Added the ability to obtain an Interpolation via a String

This is to make it easier to decide how to interpolate a value via a file.  See the new javadoc for the naming conventions you can use.

I also changed the `InterpolationTest` to use the new static `Interpolation.get()` method.  Works like a charm.  Also, the new method doesn't use reflection, just a big `ObjectMap` like the `Colors` class.  (You can also add your own `Interpolation`s.)