Expose GEOSLargestEmptyCircle

[mdavis]

GEOS has recently ​enhanced LargestEmptyCircle so that it works correctly with polygonal boundaries. See the ​JTS PR for more information. It would be nice to expose this in PostGIS.

Currently the LEC functionality is accessed via ST_MaximumInscribedCircle when the input is non-polygonal (lines and/or points). I suggest exposing it directly via a new function ST_LargestEmptyCircle(geom, boundary, tolerance). This has the advantages:

• it provides an explicit boundary constraint polygon. This will support queries such as "Find the furthest point in a jurisdiction from all roads".
• it will allow polygonal obstacles (in the future, once the underlying algorithm is enhanced)
• it allows more focussed documentation and examples

The boundary is optional; if not provided the convex hull of the obstacles is used. Perhaps the function signature should allow the boundary parameter to be optional? Or else it can be allowed to be NULL.

The existing ST_MaximumInscribedCircle behaviour can be left unchanged for backwards compatibility.

[pramsey]

Done at 90de06f418a4639f9d38b2ca8c1c41b4d8b19dda