[COOT] a request

[Eleanor Dodson]

When converting from CA to Main chain it would be wonderful to just have 
to select a CHAIN - not click on the first and last residues..
In that I cant think of any time when you would want to only convert a 
limited span of residues..

It is part of a larger request to allow many functions to be specified 
by residue identifier, rather than clicking

eg real space reefine from /A/10 to /A/17
   set occupancy to 0.0 for /A/10 to /A/17

etc etc

Eleanor