ST_AlphaShape and ST_OptimalAlphaShape need more examples

[robe]

I was trying to add some of the same examples that are in ST_ConcaveHull to ST_AlphaShape and ST_OptimalAlphaShape.

Sadly for ST_AlphaShape all my attempts ended in Empty Geometry.

I had much more success with ST_OptimalAlphaShape, but still not quite as good with the examples as ST_ConcaveHull.

I think it's mostly my understanding of they extra arguments and the docs aren't clear enough (in ours or CGAL site) for me to follow.

I also don't think these functions work with anything but Points as all my attempts at feeding in a polygon ended in Empty geometry. Need a note about that if that is true.

[Regina Obe <lr@…>]

In 0e74951/git:

More examples of alpha shapes. Fix arg names. References #5146

[robe]

I did this already.