Create Pcu abstract class and make OsmoPcu inherit from it

This base class will be used to describe the required accessors for all
PCU objects.

It is introduced in this commit and will be further used in the future
when adding a Dummy PCU object which will be used by NanoBts object.

Change-Id: Ia3fd4551d1f2932362f99f7d44d65f8ae4fd1979
2 files changed