Add Minigui Study mode support to Python engine

[tommadams]

We need to add support for the following required GTP commands:

• playsgf
• select_position

And optionally:

• prune_nodes
• winrate_evals

[amj]

python engine i think also needs support for value_init_penalty flag.

[sethtroisi]

@tommadams I believe this is done?

[tommadams]

The Python engine only supports basic Minigui functionality: self-play and vs human (enough for the Coral TPU demo). The training features don't work at all.