* Nonchiral atoms should be removed from StereoGroups
This fixes a bug where atoms which have no chirality were left in StereoGroups
In order to make this work, ROMol::setStereoGroups() needed to be made public.
That shouldn't be a problem since it doesn't change connectivity.
* support CXSmiles extensions in the hashes
* initial tests for that
* some cleanup
* copyright header cleanup
* minor refactoring
* call out the changes in the release notes
* extension and more testing
* add python wrappers