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
Explicity cast start, step to int in regular_grid
Merge pull request #852 from jni/rgrid-int-steps

Explicity cast start and step to int in `regular_grid`
Merge pull request #844 from odebeir/add_rank_sum

Add sum filter to rank filters (minor increment)