Use separate cutoff for bonded and non-bonded interactions

[fweik]

Because espresso finds bond partners by an index, and not by spatial neighbor search it is sufficient for bond partners to be present (real or ghost) on the node where a bond is evaluated. Hence the cutoff for bonds needs only to be considered for the halo size, not for the cell size. This can lead to considerable performance increase: e.g. currently adding a dihedral bond doubles the cutoff, and hence the cell size. This causes an 8-fold increase in the number of pairs that are considered in the short range pair finding. It would actually be sufficient die just use a two-cell halo and leave the cell size as is, which would not affect the performance of the hot loop.

This is also required to be able to handle multi-body bonds without the need of a reduction after the force calculation, because by selecting an appropriate halo size for the bond types present (1, 2 or 3) allows for evaluating bonds on ghost particles without influencing the performance of the pair loop.

[fweik]

Furthermore the bonded cutoff can then be ignored on a single node and for the n^2 cell system, because in either case all particles are always present.

[jngrad]

Several points have already been addressed in the upcoming 4.2.0 release, while the cell size remains an issue:

• the bonded cutoff and non-bonded cutoff are now treated separately in the short-range loop https://github.com/espressomd/espresso/blob/ef9f4979ee3ab753f44e803147fb9ae75ace2ffc/src/core/short_range_loop.hpp#L47-L53
• the bonded cutoff is now ignored on 1 MPI rank https://github.com/espressomd/espresso/blob/ef9f4979ee3ab753f44e803147fb9ae75ace2ffc/src/core/interactions.cpp#L59-L63
• the bonded cutoff is still used for the skin, and hence for the cell size via mpi_set_skin() and cells_re_init() https://github.com/espressomd/espresso/blob/ef9f4979ee3ab753f44e803147fb9ae75ace2ffc/src/core/integrate.cpp#L448-L451