add rank filter sum
shorten line
fix:rank.sum docstring
fix:rank.sum docstring
fix:rank.sum docstring
fix:indent
add:rank.sum_percentile
add:rank.sum_bilateral
fix:rank.sum docstrings
fix:docstring in sum_bilateral
fix:rtank.__init__
fix:indentation
Merge pull request #844 from odebeir/add_rank_sum

Add sum filter to rank filters (minor increment)