Specify spacegroup symmetry in vop cover command

[Tom Goddard]

Currently the vop cover command can only get symmetries from the map. The EDS maps fetched in O format don't have the symmetries. Downloading an EDS map of 1a0m in CCP4 format also did not contain symmetries (although the could be derived from the ispg header field). I'm not sure how I ever got an x-ray CCP4 map containing symmetries to test vop cover.

Would be helpful to be able to specify using the PDB model space group and unit cell symmetries in the vop cover command. Also would be nice to be able to specify space group symmetries in the volume command symmetry option, and the sym command for generating a unit cell.