Nano-cat

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* CAT_ has been split into 3 seperate packages (see https://github.com/nlesc-nano/CAT/issues/39):

* CAT_: A collection of tools designed for the automatic construction of composite chemical compounds.
* nano-CAT_: A collection of tools for the analysis of nanocrystals.
* data-CAT_: A databasing framework for the Compound Attachment Tools package (CAT_).

* Docstrings have been changed into NumPy style.
* Added typehints.
* Added the CAT.SettingsDataFrame and CAT.SettingsSeries classes.
* Added more tests.
* Cleaned up all input-parsing related modules.
* Custom function groups (*i.e.* SMILES_ strings) can now be specified in the input
under the optional.ligand.functional_groups key (see https://github.com/nlesc-nano/CAT/issues/13).

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* Added an interface between MongoDB_ and the CAT.Database_ class (see https://github.com/nlesc-nano/CAT/issues/11).

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* All raw input scripts are now stored in the structures.hdf5 file
(see: https://github.com/nlesc-nano/CAT/issues/36).

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* Split CAT_database.py into database.py and database_functions.py.
* Unoptimized starting structures are now exported to the database.
* Added the sphinx autosummary extension.

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* Improved interaction between the database and BDE module.
* Cleaned up BDE module.
* HDF5 indices are now always sorted when itneraction with the database.

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* Numerous bug fixes.
* A couple of code-style changes.