#206 Chain creation issues

[wb104]

• We need some good documentation of the residue type codes and how they work. Basically all residues are given by atype string that is unique across all types. So the one-letter code 'A' corresponds to 'A' for RNA, 'DA' for DNA, and 'ALA' for protein. E.g. 'GUA' is not recognised as wither RNA or DNA.
• When and how one-letter codes are used and recognised needs checking and documentation. You can certainly create a protein chain from a string of one-letter codes. But I think that otherwise chain creation is given as a sequence of full codes, which would mean that 'ACGT' and 'A C G T' both gave you RNA chains. The interplay between the molecule type pulldown and the value of the sequence needs to be checked, decided on, and documented.
• We must remember in all contexts that sequences can contain non-standard residues (with no one-letter code), can be of mixed type, and can contain non-polymer residues like glucose, calcium, or NADP.