parallelized the ray trace

now there's numprocs*beam_res rays being traced uniformly across the
beam. There is no redundancy in calculations now, the most expensive
part of the ray-trace has been parallelized.

Also, random number generation seeding is improved.
Added one step of diffusion equation
Merged branch from intensity-diffusion
small fix
not sure?
Added and tested diffusion to intensity map

Only 100 steps currently. These do cause enough of a perturbation (~1%)
to help. Can also consider adding to input file
added intensity-diffusion to input file
changed the width of the super gaussian
added the intensity diffusion steps to tracer

foolishly not implemented previously,
shame