initial implementation of new features class
add Features.from_data
most adaptations to proc_image_features
a few more changes to proc_, some to extract_
remove tail fixing stuff; min_dist should be enough
most of a rewrite of np-divs

The new tests almost all fail, though, so this isn't working yet.
whoops, kl was backwards
divs cache: actually overwrite pre-existing ones
fix kl results in test files
everything's a meta-estimator! saves a bunch of work in some cases
update doctests for _parse_specs
kl, alpha: clamp results to >= 0
estimate_divs was filling in the matrix transposed
estimate_divs CLI: actually use the --n-proc option
test data: fix thresholding change for kl/alpha
loosen test threshold a bit
fix l2 a bit; now all divergence tests pass!
since the posix check isn't in np_divs anymore, do it in ForkedData.__init__
change sdm hierarchy a bit
better Features.__getitem__; move some utility functions