Add support for memory usage estimates from PixelDistribution / PixelData

[tskisner]

Once a PixelDistribution object is created, the number of elements stored on each process (or each node, once shared memory is used) is fully specified. This information can be used to estimate the aggregrate and per-node memory usage. Adding this feature would allow us to estimate the map-domain memory allocation needed before doing that allocation and at least inform the user or perhaps raise an exception if there is insufficient memory available. Unfortunately we can't compute this without going through the pointing once (so we cannot do it at the start of the job), but this information would still be useful later in the workflow.