BUG FIX: in module_master.f90:init_loadb, a domain with 0 photon could be assign…

…ed cpus (when correcting for rounding errors). This caused crash later when trying to collect photons for these cpus. The fix is (1) use 'int' instead of 'nint' for the rounding, so there is never an over-estimate; (2) assigning left-over cpus to the domain which has most cpus (this cannot be a bad bet).