Implement missing predicate functions

Here I implement the following GDMS predicate functions in h2spatial-ext:

* ST_Covers
* ST_IsRectangle
* ST_IsValid
* ST_DWithin (replaces ST_IsWithinDistance for consistency with PostGIS)

(Note that the remaining GDMS predicate functions

* ST_Contains
* ST_Crosses
* ST_Disjoint
* ST_Equals
* ST_Intersects
* ST_Overlaps
* ST_Touches

have already been implemented by @nicolas-f in h2spatial, and that

* ST_IsWithin

has been replaced by ST_Within (SFS-compliant) in h2spatial.)