Adding Interfaces to families of operators; permits much cleaner UDFs
* New interface IOperatorAWithB for (geom, geom) -> geom
* Applied it to Intersection, Difference, SymmetricDifference, and Union
More to come.
Minor fixes: error messages on import; gitignore artifacts; make NumberUtils available to UDFs
toString() on Geometry and OGCGeometry
CombineOperator replaces IOperatorAWithB.java
Merge pull request #42 from mrflip/operators_have_interfaces

Families of operators have common interfaces