Introduce ratio operation

Just like in Physics, the ratio of 2 things in the same unit, should be
without unit.

Example use case:
- ratio of a Depth by a Depth (eg. ONE_PLY) should be an int.
- ratio of a Value by a Value (eg. PawnValueEg) should be an int.

Remove a bunch of useless const while there.

No functional change.
This is better to do during tables initialization, not at runtime. With ONE_PLY == 1 compiler optimizes divide away, but not in general case and anyhow conceptually it is better to handle this at table init and only lookup the table at runtime.