Improved correct command and assignment of multimapping reads

This release improves the memory footprint and speed of the correct command of bustools.

The bustools count command adds the --em option that estimates gene abundances using an EM algorithm for reads that pseudoalign to multiple genes.

Note that the --multimapping option splits the read counts evenly across all genes, whereas the EM algorithm gives a more statistically valid answer. The two options are mutually exclusive.