Export .pml script to visualize restraints in pymol

[rvhonorato]

This PR adds an optional argument --pml <output_f> to the tbl, ti, unambig-ti and restraint subcommands.

When this optinal argument is defined the restraints will also be outputted as a PyMOL script file (.pml) which can then be loaded together with the input for a convenient visualization of the restraints:

example:

cd example/
haddock-restraints tbl restraints.json --pml visualization.pml > ambig.tbl
# and then
pymol 2oob.pdb visualization.pml

or the true-interface with the ti subcommand:

Imporant to note is that this will only take CA as anchor atoms - i'll work on a better logic in a different PR - but this one should work for protein-related things