v0.13 "Timestream"

This update is the debut of the timestream view, which makes it easier to time travel between branches of history.

It also solves major issues preventing branches from being very useful--some crashes, and one very important bug that prevented branches from entirely loading between sessions.

There is now a LiSE message board.

Other changes:

• rename "avatar" to "unit"
• Nodes now support the `.user.only` property, just like characters' `.unit.only`
• Rename some Engine methods to use snake_case
• numpy is no longer required for the LiSE core (will speed up IPC, though)
• Improve interprocess communication some more, particularly when time travelling between branches
• Tab indentation, with yapf formatting
• Drop support for Python 3.6