FEAT: Implement the concept of the power of an action

[tturocy]

Overview

Based on having the set of plays consistent with various game objects, we are in a position now to implement the concept of the "power" of an action

Outline specification

• Implement in C++ GameRep::GetPower(GameAction), which for the moment will return a container representing the power of the action.
• Implement in Python Action.power, a property which does the same
• Add appropriate test cases to the test suite
• Add to the documentation index for Python