counts with many features/options should result in many files

[luispedro]

Not clear exactly on the desired behaviour/API. But here is a proposal:

counts = count(mapped, features=['A', 'B'], multiple=[{dist1}, {all1}])
write(counts, ofile='counts.{features}.{multiple}.txt')

would result in 4 files: counts.A.dist1.txt, counts.A.all1.txt, counts.B.dist1.txt, counts.B.all1.txt.

[unode]

This should also cover subfeatures. At the moment specifying multiple subfeatures adds two additional columns to the output.