Remove the Ensemble.batch use_map kwarg in favor of an implicit check for divisions

[dougbrn]

use_map indicates whether to use a map_partitions call or just a general groupby when grouping lightcurves. The original motivation for this keyword was due to the fact that we didn't have a way to know if lightcurve cohesion would be true or not. I believe we're at the stage where if divisions are known, there's relatively high confidence that lightcurve cohesion is true (the only exception being if someone loads a non-lightcurve cohesive dataset into a dask dataframe from parquet, setting divisions using the calc_divisions flag, and then loading that dask dataframe into tape). I propose we do away with use_map and instead check if divisions are known, and in the case they are just automatically do a map_partitions call. The general groupby can be saved for the case where divisions are not known, as it's a more general/robust call.