Add "byChain" option to fitmap command

[Tristan Croll]

Someone from my building came to me yesterday, asking for help in setting up a chain-by-chain rigid-body refinement of their model into a new EM map. They were using a different package, and struggling with using its GUI interface to define the individual rigid bodies. I showed them how they could easily get the same result in ChimeraX using a quick Python loop over chimerax.map.fit.fitcmd.fit_atoms_in_map, which they were happy with... but it got me thinking that this would probably be a feature that many people would like to have in the command interface.

Something like fitmap #1 inMap #2 byChain true

[goddard@…]

Good idea.  You can already do this if you do one extra step.  Use the "split" command to split make each chain of the structure a separate model, then use the fitmap command eachModel option. Or better use the fitmap command sequential option which takes into account clashes between the different chains.

While it would be more convenient to not do the split, this is a common issue in Chimera and ChimeraX that many actions that work at the model level don't work at the chain level.