Initial doc for watershed_sequence
Implement watershed_sequence
Fix all bugs in watershed_sequence
Implement minimum_seed_size, improve documentation

The `minimum_seed_size` kwarg to morpho.watershed was not actually
implemented. This commit adds that, and additionally updates the
nearly nonexistent doc to comply with numpy guidelines.
Further bug fixes for watershed_sequence
Remove small seeds only when min size > 0
Merge pull request #3 from jni/2d-sequence-watershed

perform watershed plane-by-plane instead of in 3D