adds support for backward actions in MolBuildingEnvContext,
adds tests for MolBuildingEnvContext that check that backwards masks are correct,
adds a toy atom environment where the reward is simply the number of rings in a carbon-only molecule of up to 6 atoms.
fixes a bug in GraphBuildingEnv.parent, whereby 1-node graphs with attributes would have their parents misenumerated (which hadn't manifested itself because until now we didn't have contexts with more than 1 node attribute).
This PR:
MolBuildingEnvContext,MolBuildingEnvContextthat check that backwards masks are correct,GraphBuildingEnv.parent, whereby 1-node graphs with attributes would have their parents misenumerated (which hadn't manifested itself because until now we didn't have contexts with more than 1 node attribute).