v0.5.4

Optionally show the U stat (uncertainty).
Spawn Leela with cwd as its own directory - should work better with Leela's own lc0.config file, if present.
An unrelated major code refactor. As ever, some chance this broke something, please report problems.

Note that, if you were previously using a relative (not absolute) path to specify your weights file, this will now break. Either change it to be relative to Leela rather than Nibbler, or use an absolute path.