Re-orient parallelization strategy.

[cwwalter]

In the current version of imSim, we pass a list of objects and a sensor to each core in a multi-processor pool. So each sensor is done with all of its objects sequentially and each sensor is done independently of each other and the image itself is returned.

In the redesign, instead we will farm out each astronomical object independently (thorough something like MP or MPI ) and the returned information will be a list of photos from that object. Then all of the photons will be gathered together in a pool and rained down on the sensor.

This has many advantages including but not limited to the ability to precisely track photon provenance if desired, better physical realism for sensor effects such as B/F, better interfacing with batiod, and the ability to consider the local plane as a whole.