Data-cat

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* *placeholder*.

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* Replace ``pd.DataFrame.append`` with ``pd.concat``.
* Fix size-1 molecular dataframes not being correctly updated.

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* Deprecate usage of ``Molecule.get_formula`` in favor of a PLAMS <=1.5.1-based backport.

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* Add a new ``PDBContainer`` method for creating rdkit molecules.
* Expose the ``index_to_pandas`` function.
* Various fixes.

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* Fixed an issue where not all properties were copied from the .csv to the .hdf5 file.

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* Removed the .yaml job recipe storage component of the `Database` class.
* Added a function for validating property Groups.
* Cleaned up the `Database` class.
* Ensure that all content written to the .csv files is also written to the .hdf5 file.
* Pass the `"logger"` group directly to the `hdf5_log` functions.
* Added `prop_to_dataframe()`, a function for converting property Datasets into DataFrames.
* Added set-based methods to `PDBContainer`: `.union()`, `.intersection()`,
`.difference()` & `.symmetric_difference()`.
* Added `PDBContainer.concatenate()`.