* Implementation of SGroups
* remove sample files test
* update gitignore with test outputs
* fix RevisionModifier
* re-enable tests
* backup commit; things seem to work so far
* some refactoring; obvious s group tests pass now
* more refactoring
* everything now out of the public API
* not sure why this was still in there
* rename functions; all tests now pass
* remove getNextFreeSGroupId; readd comment in copy SGroups
* clang-format
* squash-merge current master
* squash merge master
* Address comments on PR
- Update to current master.
- Move SGroup parse time checks to SGroupChecks namespace.
- Store SGroups in ROMOl as vector<SGroups>.
- SGroup methods return referenes instead of pointers.
- Use atom/bond/sgroup indexes for properties instead of pointers.
- Have SGroups inherit from RDProps; move properties to RDProps.
- Remove trivial/unused methods.
- Add a link to the SD specification atop SGroup.h
