Ensure consistent evaluation perspective across the code base

[knutole]

There are three ways to return evaluations:

• From the perspective of whose turn it is (ie. +5 is advantage to the side whose turn it is)
• From the perspective of White always (ie. +5 is advantage White, no matter whose turn it is)
• From the perspective of Black always (ie. +5 is advantage Black, no matter whose turn it is)

Currently there is a happy mix in the codebase:

• Stockfish engine uses Turn perspective.
• get_evaluation() returns White perspective always
• get_top_moves() returns White perspective by default (but when https://github.com/py-stockfish/stockfish/pull/16 is merged, it will use new turn_perspective global option, which defaults to Turn.)
• get_wdl_stats() returns Turn perspective

Suggested solution:

• Use turn_perspective global option for all functions, default to True (ie. Turn perspective).
• Problem: backwards compatibility for above functions that use White perspective.

[kieferro]

Good point. We should definitely try to standardize that. If there's no other way, we could also consider deviating a bit from backwards compatibility and doing a major version bump with bigger changes.