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.
`Depth * Depth` does not compile, for the reasons I've already explained. The compiler has to resolve an ambiguity between 3 possible `operator*`, because of stupid C++ rules allowing implicit conversion to int:
* user defined: `Depth operator*(Depth,int)`
* user defined: `Depth operator*(int,Depth)`
* C builtin operator*(int,int)

I told you this implicit int conversion is a can of worms. But you didn't believe me.