Bug fixed to relax boundary, Field3D::min and max

o The relax() boundary condition modifier didn't have the
  default relaxation rate set, so "relax(neumann, 10)"
  would work fine, but "relax(neumann)" used an uninitialised
  rate.

o Field3D::min and max functions were modified to improve OpenMP
  performance, but meant they also included guard cells, boundary
  regions etc. Changed back to only include the core cells
Updated lapd-drift test case

Now working again. Reduced Z resolution to 32 for shorter
test runs. Should be increased for production runs.