Crem

- add a module to enumerate compounds based on a seed molecules using scaffold or analog modes

- x2 code speed up
- fix pickling mol properties in multiprocessing
- add a tutorial on machine learning and CReM

- add support of custom parameters which can be used for fragment selection (**kwargs in mutate_mol, grow_mol, link_mols). A user can add custom parameters (e.g. pharmacophore features count, TPSA, etc) for each fragment in additional columns of a database and use these columns for fragment pre-filtering.

- fixed leakage of resources due to not closed pool in generators

- added jupyter notebook with examples
- fix mutate_mol generator for Python 3.7 and later

- fix link operation to return all possible links irrespectively to the order of linked molecules