Change Interpolation interface to an abstract class

org.apache.sis.image.Interpolation is currently an interface. There is some benefits to make it an abstract class instead. It would allow the addition of a package-private method for handling special cases that we do not want to expose in public API (those special cases are needed for resolving some exceptions observed with Visualization). Another potential (but not verified) benefit is a potential performance gain. The Interpolation.interpolate(…) method can be invoked million of times for each image, and benchmarks report slightly faster method calls on abstract classes compared to interfaces.