Data-cat

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* Updated the hdf5 logging system.
* Moved all dtype-related functions and data to `dataCAT.dtype`.
* Cleaned up the global namespace.
* Added the `PDBContainer.index` attribute.
* Create the `index` Dataset when calling `PDBContainer.create_hdf5_group()`.
Aforementioned Dataset is used as dimensional scale.
* Provide users more control over the keyword arguments used for creating datasets.
* Fixed an issue where`PDBContainer.to_molecules()` would set values as numpy scalars
rather than using its builtin counterparts.
* Added an API for creating and managing datasets for containing quantum-mechanical
properties in hdf5 format.
* Cleaned up the backwards-compatibility checks in `dataCAT.create_database`.

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* Fixed bug with the `PDBContainer` hashing system.
* Moved part of the hdf5 database creation process to `PDBContainer.create_hdf5_group()`.
* Added `PDBContainer.keys()` and `PDBContainer.values()`.
* Moved a number of test files to `dataCAT.testing_utils` and `dataCAT.data`.
* Ignore flake8 `N806`.
* Added a test for building wheels.
* Added `h5py` to `install_requires`.
* Check for the presence of `rdkit` when running `setup.py`.
* Added `PDBContainer.validate_hdf5()`.
* Execute `.validate_hdf5()` automatically when `.to_hdf5()` or `.from_hdf5()` crashes.
* Moved `hdf5_availability()` from `Database` to `dataCAT.functions`.
* Cleaned up the global namespace.
* Added `libver="latest"` to `Database.hdf5`.

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* Overhauled the .pdb storage system.
* Introduced the `PDBContainer` class.
* Renamed `dataCAT.database_functions` to `dataCAT.functions`.

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* Updated the documentation (see https://github.com/nlesc-nano/CAT/pull/123).

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* Store the ``__version__`` of CAT, Nano-CAT and Data-CAT in the hdf5 file.
* Store the content of the .csv file also in the .hdf5 file.
* Reach a test coverage of >= 80%.

*****
* Moved from travis to GitHub Actions.
* Enabled tests for Python 3.8.
* Added tests using ``flake8`` and ``pydocstyle``.
* Removed the unused `requirements.txt` file.
* Cleaned up `setup.py`.
* Cleaned up the context managers; removed the `MetaManager` class in favor of `functools.partial`.
* Cleaned up the ``DFCollection`` class; renamed it to ``DFProxy``.