Pdbx2df

Last version under the name **pdbx2df**. Now it is renamed as **MolDF**.

Future installation by `pip install pdbx2df` will install `moldf` as well.

Fixes

- Fixed the problem that `PDBDataFrame` `.atoms` failed for `PDBx` DataFrame.

From this version on, changelog is kept for each new version, compared to the previous version.

New features
- Parsing `mol2` file header lines starting with `` in `read_mol2`.
- `radius_of_gyration` can be accessed as a property in `PDBDataFrame`.
- `rmsd` method added in `PDBDataFrame`.
- Can feed `PDBDataFrame` with the PDBx `_atom_site` DataFrame now.

Fixes
- Corrected `center_of_mass` calculation in `PDBDataFrame`.

Docs
- Cleaned up empty lines in all function and method's docstrings.
- Added a quick start section for calculating RMSD, radius of gyration, and distance matrix.