Add option to remove low PLDDT residues from domain assignments on AlphaFold structures

[JudeWells]

Add some functionality to the post-processing which modifies domain assignments to remove residues which have low PLDDT (which is the b-factor column of PDB files generated by AlphaFold).

[JudeWells]

I'm working on this currently

[alex-hh]

in case of any use i did some refactoring to support this here (was intended for unidoc but also compatible with chainsaw): https://github.com/JudeWells/chainsaw/compare/unidoc