REF: rename DateOffset -> RelativeDeltaOffset

[jbrockmendel]

It used to be that DateOffset was the base class and all other offsets subclassed it. Now BaseOffset is the base class that everything subclasses, RelativeDeltaOffset holds the old-DateOffset logic, and DateOffset is a thin wrapper around RelativeDeltaOffset to make old isinstance(obj, DateOffset) checks work.

It'd be nice if we could expose RelativeDeltaOffset without wrapping it as DateOffset.

[mroeschke]

DateOffset is in the public API so I guess it would need to be deprecated if RelativeDeltaOffset were to fully replace it.