Expose Go model backbone and atomname in add_virtual_sites

[Lp0lp]

Would be useful to expose the backbone and atomname variables in add_virtual_sites, to more easily control where to place the GoVirt bead and how to name it, via the CLI.

vermouth-martinize/vermouth/rcsu/go_vs_includes.py

Line 63 in dbe0d76

def add_virtual_sites(self, molecule, prefix, backbone='BB', atomname='CA', charge=0):

[pckroon]

Sure, PR welcome. I also see no reason why not to expose the charge argument to the CLI as well then.