Resolve deprecation of selem argument (now footprint) in skimage.morphology.dilation

[davedgd]

Recent versions of scikit-image have completely deprecated the selem argument in favor of dilation:

skimage.morphology.dilation Complete deprecation of selem parameter

This minor patch to xrai.py resolves the issue on line 126.

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[davedgd]

@mantasu: Makes sense -- updated all three references from selem to footprint -- thanks!

[tolga-b]

Thanks for submitting this fix!