GP Builder ASN dihedral angles options for users

[gitoliver]

User feedback is that we should have these options:

A) Retain (not restrain) initial glycosite orientation (freeze asn X1 and X2 at initial values). (Currently available)
B) Prioritize shapes close to initial glycosite orientation (keep asn X1 and X2 values within +/- 15 deg of initial values)
C) Prioritize glycosite shape diversity (vary asn X1 and X2 angles through all common values)

Issues:
Code does not know what the input dihedral angle rotamers are. It just sets them.

[gitoliver]

C) Prioritize glycosite shape diversity (vary asn X1 and X2 angles through all common values) -> Tobias says this is current behaviour.