Crem

- add link to crem.imtm.cz to README
- change default separator in fragmentation.py to tab
- fix description of arguments for grow_mol
- fix output of molecules identical to the input structure

- add to mutate and grow functions treatment of topologically equivalent atoms

- 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