Basic support for 'z of ...' selections

Add basic support for selections of type 'z of ...', where "z" can in
principle be any keyword and "..." any expression.  However, currently
the only syntax that is actually supported is '[xyz] of co[gm] of ...',
because
 1) this is the original case that was discussed in gmx-users some time
    ago,
 2) only the x, y, and z keywords satisfy the constraints that the
    simplest possible implementation requires,
 3) there is no context that would accept multiple values as produced by
    an expression like 'z of resnr 1 to 3', and
 4) there is no mechanism to detect that 'z of atomnr 1' actually
    evaluates to a single value.
Some of these limitations can be lifted in the future with reasonable
effort, though.

Change-Id: I6b87ce8d5c93e1ad05722dd59a66375b52b8e363