* Fixes#2277
* changes in response to review
the big one is to move the PXA parser into the normal mol file parsing
* move the PXA changes to the writer as well
* SCN actually only needs 7 characters
* add test
* fixes in response to review
* handle blanks (instead of zeros) in the counts line.
The ctfile.pdf doc says we should do this
* Make the SGroup reader more robust w.r.t. bad data
The current behavior leads to uncaught exceptions when a line is too short.
This should clear that up so that we always throw the usual FileParseException
* make error messages a bit easier to read
* 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
