Add "safety margin" argument for ICE

[mimakaev]

basically:
cooler.ICE(..., nnz_safety_margin = 0) # suggested value ~3-4
if 2 * cooler.info["nnz"] / cooler.info["nbins"] < nnz_safery_margin * min_nnz:
do_nothing_and_dont_do_ICE

if nnz_safery_margin = 1, and cooler.ICE is performed, then exactly at the margin about 1/2 of the bins will be filtered out, which is bad. If nnz_safety_margin=3, then maybe only 5-10% of the bins will be filtered out, and if the dataset is more sparce, ICE will be aborted.

[gfudenberg]

relatedly, should the actual value that the mad_max filter thresholds at be reported in the balance.py stats? e.g. 'mad_max_cutoff':cutoff
this could be useful for debugging balancing.

[gfudenberg]

bumping this-- for balancing, it would be useful to add an easy way to see what thresholds in terms of counts a given mad-max value would return for a given cooler. it would also be useful to save the #counts as metadata.

[nvictus]

See #210