Point extents func to fit marker size + stroke

[eaton-lab]

I updated the .extents() for Point Marks to return extents of its markers as msize / 2 + stroke, unless marker type is "rNxM", where it gets height and width from the marker. I'm not sure if there are other non-symmetric markers that need to be accommodated. I've had something like this implemented in toytree for a while and it seems to work well. Thought it would be good to share. I believe this addresses Issue #164

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This shows the new extents seem quite accurate by suppressing the padding and margin: