Refactor, test CharacterSkill._psionic_skill_score
Refactor two CharacterSkill methods

Refactor the following CharacterSkill methods:
* _mental_skill_score
* _physical_skill_score

As part of the refactoring, create the following two methods:
* _effective_points_mental
* _effective_points_physical
Test method CharacterSkill._mental_skill_score
Test method CharacterSkill._physical_skill_score
Make pylint happy

Consolidate redundant `pylint: ignore=W0212` directives.