The real-space residual RSR, as described by Branden and Jones,
               evaluated over all atoms in the monomer.
                     sum|pobs - pcalc|
               RSR = ---------------------
                     sum|pobs + pcalc|
               pobs  = the density in an "experimental" map
               pcalc = the density in a "calculated" map
               sum is taken over the specified grid points
               The details of how these maps were calculated should be described
               in _struct_mon_details.RSR. The sums are taken over all map grid
               points near the relevant atoms. The radius for including grid
               points in the calculation should also be given in
               _struct_mon_details.RSR.
               Ref: Branden, C.-I. & Jones, T. A. (1990). Nature, 343,
                    687-689.