FAQ example: creating new chain/segid identifiers

[orbeckst]

Show how to create custom segid identifiers (so that one can write a PDB file with custom ChainIDs).

See also MDAnalysis/mdanalysis#1105 and MDAnalysis/mdanalysis#947

[orbeckst]

@richardjgowers said:

The problem here is assigning new names won’t create new segments. Instead this should work:

newseg = U.add_Segment(segid=‘T’)
U.select_atoms('bynum 1201:2400').residues.segments = newseg