purge PDB option available

Now it's possible to create a PDB file purged of the HETATM and ligands found, i.e. ligands which match -- with some tollerance (note tolerance can be disabled with the "safe" option)-- the valid ligands in the binding MOAD database.
This is activated with the --purgePDB option and creates a new file named "<original structure name>_clean.pdb"

First release

v1.0

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