Addresses several minor warning messages during the build (#1935)

* Address minor compilation warning messages

Addresses trivial compilation warning messages:

    [ 11%] Building CXX object Code/GraphMol/CMakeFiles/GraphMol.dir/AddHs.cpp.o
    rdkit/Code/GraphMol/AddHs.cpp:497:9: warning: unused variable 'dblBond' [-Wunused-variable]
      Bond *dblBond = nullptr;
            ^
    1 warning generated.

    [ 11%] Building CXX object Code/GraphMol/CMakeFiles/GraphMol.dir/Chirality.cpp.o
    rdkit/Code/GraphMol/Chirality.cpp:1738:62: warning: unused parameter 'mol' [-Wunused-parameter]
    bool isBondCandidateForStereo(const Bond *bond, const ROMol &mol) {
                                                                 ^
    1 warning generated.

    [ 44%] Building CXX object Code/GraphMol/CMakeFiles/graphmolMolOpsTest.dir/molopstest.cpp.o
    rdkit/Code/GraphMol/molopstest.cpp:4694:12: warning: unused variable 'nm' [-Wunused-variable]
        ROMol *nm = MolOps::renumberAtoms(*m, nVect);
               ^
    rdkit/Code/GraphMol/molopstest.cpp:6941:16: warning: unused variable 'm' [-Wunused-variable]
            RWMol *m = SmilesToMol(smiles);
                   ^
    2 warnings generated.

    [ 42%] Building CXX object Code/GraphMol/CMakeFiles/graphmoltestPicklerGlobalSetting.dir/testPicklerGlobalSettings.cpp.o
    rdkit/Code/GraphMol/testPicklerGlobalSettings.cpp:257:14: warning: unused parameter 'argc' [-Wunused-parameter]
    int main(int argc, char *argv[]) {
                 ^
    rdkit/Code/GraphMol/testPicklerGlobalSettings.cpp:257:26: warning: unused parameter 'argv' [-Wunused-parameter]
    int main(int argc, char *argv[]) {
                             ^
    2 warnings generated.

    [ 55%] Building CXX object Code/GraphMol/FilterCatalog/CMakeFiles/FilterCatalog.dir/FunctionalGroupHierarchy.cpp.o
    rdkit/Code/GraphMol/FilterCatalog/FunctionalGroupHierarchy.cpp:176:20: warning: unused variable 'NUM_FUNCS' [-Wunused-const-variable]
    const unsigned int NUM_FUNCS =
                       ^
    1 warning generated.

    [ 84%] Building CXX object Code/GraphMol/StructChecker/CMakeFiles/StructChecker.dir/ReCharge.cpp.o
    rdkit/Code/GraphMol/StructChecker/ReCharge.cpp:405:17: warning: unused variable 'atom' [-Wunused-variable]
        const Atom &atom = *Mol.getAtomWithIdx(i);
                    ^
    1 warning generated.

    [ 84%] Building CXX object Code/GraphMol/StructChecker/CMakeFiles/StructChecker.dir/Tautomer.cpp.o
    rdkit/Code/GraphMol/StructChecker/Tautomer.cpp:61:12: warning: comparison of integers of different signs: 'int' and 'unsigned int' [-Wsign-compare]
        if (-1 == ti || -1 == tj) continue;
            ~~ ^  ~~
    rdkit/Code/GraphMol/StructChecker/Tautomer.cpp:61:24: warning: comparison of integers of different signs: 'int' and 'unsigned int' [-Wsign-compare]
        if (-1 == ti || -1 == tj) continue;
                        ~~ ^  ~~
    2 warnings generated.

    [ 88%] Building CXX object Code/GraphMol/RGroupDecomposition/CMakeFiles/testRGroupDecomp.dir/testRGroupDecomp.cpp.o
    rdkit/Code/GraphMol/RGroupDecomposition/testRGroupDecomp.cpp:428:9: warning: unused variable 'res' [-Wunused-variable]
        int res = decomp.add(*mol);
            ^
    1 warning generated.

    [ 96%] Building CXX object Code/SimDivPickers/CMakeFiles/testSimDivPickers.dir/testPickers.cpp.o
    rdkit/Code/SimDivPickers/testPickers.cpp:19:10: warning: using integer absolute value function 'abs' when argument is of floating point type [-Wabsolute-value]
      return abs((double)i - (double)j);
             ^
    rdkit/Code/SimDivPickers/testPickers.cpp:19:10: note: use function 'std::abs' instead
      return abs((double)i - (double)j);
             ^~~
             std::abs
    1 warning generated.

* Close open file handles during build set-up

Under Python3, the update_pains.py file generated warning
messages like:

    == Done updating pains files
    /Code/GraphMol/FilterCatalog/update_pains.py:140: ResourceWarning:
unclosed file <_io.TextIOWrapper
name='/Code/GraphMol/FilterCatalog/../../../Data/Pains/wehi_pains.csv'
mode='r' encoding='UTF-8'>
      for smiles, name in csv.reader(open(PAINS_CSV)):
    /Code/GraphMol/FilterCatalog/update_pains.py:169: ResourceWarning:
unclosed file <_io.TextIOWrapper
name='/Code/GraphMol/FilterCatalog/pains_a.in' mode='r'
encoding='UTF-8'>
      t = open(filename).read()
    /Code/GraphMol/FilterCatalog/update_pains.py:169: ResourceWarning:
unclosed file <_io.TextIOWrapper
name='/Code/GraphMol/FilterCatalog/pains_b.in' mode='r'
encoding='UTF-8'>
      t = open(filename).read()
    /Code/GraphMol/FilterCatalog/update_pains.py:169: ResourceWarning:
unclosed file <_io.TextIOWrapper
name='/Code/GraphMol/FilterCatalog/pains_c.in' mode='r'
encoding='UTF-8'>
      t = open(filename).read()

Code/SimDivPickers/testPickers.cpp

@@ -16,7 +16,7 @@
 
 namespace {
 double dist_on_line(unsigned int i, unsigned int j) {
-  return abs((double)i - (double)j);
+  return std::abs((double)i - (double)j);
 }
 }
 void testGithub1421() {