0.20.0 - Support for processing proteins through PDBFixer

• Adds an optional pdbfixer directive to the molecules section of the YAML file through PDBFixer, a simple OpenMM-based protein structure processing tool.
• The following options are accessible through the pdbfixer directive.
  • replace_nonstandard_residues: Replace nonstandard amino acids.
  • remove_heterogens: Remove heterogens (such as ligands and waters).
  • add_missing_residues: Add missing residues from the SEQRES block.
  • add_missing_atoms: Add missing heavy atoms.
  • apply_mutations: Specify protein mutations (e.g., T315I).