CUSTOM MDCATH

[AntonioMirarchi]

This PR introduces a custom version of MDCATH while preserving the original functionality to ensure backward compatibility.

New Arguments:

• source_file and file_basename: these args enable users to create and train with their custom versions of the MDCATH dataset.
• numNoHAtoms by default is None, max number of non-hydrogen atoms in the protein structure.
• Assertions have been added to ensure that automatic downloading is restricted solely to the original version of the MDCATH dataset.

These enhancements aim to provide users with greater flexibility in dataset handling and customization, aligning with diverse research needs.