Fpu implementation

[Whojo]

Adds FPU ("First Play Urgency") inspired by Leela-Chess:

• 2 FPU strategies
  • reduction: subtract the fpu_value to the parent's value
  • absolute: directly replace new nodes value with fpu_value

Note:
FPU is not applied yet at the root node, as in game of src/mcts.jl:explore! for example

Feel free to propose any modifications.

[jonathan-laurent]

This looks great. I will merge after we run some experiments on Connect Four. :-)

[oscardssmith]

based on lc0, relative fpu that is fairly negative is probably a good default.