Verify the number of MPI processes to use for a run before launching a cluster job

As part of the httomo cluster job submission script that will be needed to be written before it goes into production at DLS, there should be some logic in the script to ensure that the number of MPI processes is compatible with the data. In particular, heavily cropped data (via previewing in the loader) could easily cause issues with having too many MPI processes compared to the shape of the data.

For example, heavily cropping the detector y dimension to be only a few slices, say 5, will inadvertently cause the recon method to only have 5 slices/sinograms to reconstruct. If the number of MPI processes is then larger than 5, this will result in an error in the recon, due to the data being split nonsensically between the MPI processes.