completely eliminate the use of subcommunicators #210
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#203 introduce a "pure-MPI" mode which used the concept of subcommunicators in lieu of multiprocessing, motivated by the idea that this mode would be more efficient at-scale / in production.
Performance at NERSC, however, has been terrible. Furthermore, reverting to using MPI (over healpix files) and multiprocessing (over objects) is still extremely slow (by factors of 20-30) when using >>8 nodes, so something is still going on with the current version of the code that I don't understand.
So this PR gets back to basics of simply using rank 0 to divide work among
ntask-1other MPI ranks. Let us hope that this works, otherwise Loa/Y3 processing will never finish!Once NERSC is back online, I'll report test results before merging, tagging, and deploying.