optionally generate separate X3D objects for each nucleotide

[m.aquilina@…]

From chimerax-users:

[T]he 3D models exported by ChimeraX aren’t grouped into individual objects per nucleotide, so I am not able to easily select nucleotides and colour them in according to their type (A, T, G or C). With the older version of Chimera, these nucleotides are exported as individual objects, but then are given identical names with randomized IDs. This means that setting the nucleotide types is a long manual process of clicking on each resulting nucleotide and naming them individually.