Allow left/right only bonding

[jonrkarr]

Some the protein residues in RESID do not have C terminals because they have modifications to the C terminus that prevent further bonding. Several changes would have to be made to accomodate this

• Change the representation of the protein alphabet so that the monomers include the C terminus and any modifications to the C terminus. With this change, the backbone for ProteinForm and CanonicalProteinForm would become None. Note, this change would make the representation of the protein alphabet different from that of RESID, which we have tried hard to preserve.
• Add attributes to indicate which monomers have terminii for left and right bonding

[jonrkarr]

We have decided not to make these changes, at least not for now, because the inability for monomers to bond in the both the left and right directions might not be immediately clear to users who will likely expect that every monomer can appear at any position. This would result in users getting what they perceive to be unexpected error messages, which might frustrate them. -Users would have to pay attention to the left/right bonding ability of subunits, which we would display in the HTML alphabet pages

• Users would see errors when they try to put subunits that can only appear at the left or right at other positions