Lise

* Automatically unload history, thus minimizing memory usage for long games
By default, this happens whenever you commit changes to the database.
* Improved support for ELiDE widgets in games
* New example sim, awareness.py, to demonstrate this
* Moved `game_start` into its own module
* Dropped support for Python 3.7 and the lz4 module
* A lot of fixes regarding thread safety and interprocess sync

* Fix the rest of the loader

* Fix a loading error when you make two keyframes and then start the engine before the first

* Seek bar for time travel in ELiDE, and some other minor UI improvements
* Vastly improved performance in ELiDE for large worlds
* History queries no longer require the whole of history to be loaded, and are faster
* SQLAlchemy is now a hard dependency
* numpy is required again
* Save the state of the randomizer every time you use it, not just on time travel
* Rule stepper now works when you reopen the sim
* New example sim, wolfsheep.py
* `enforce_end_of_time` option, so it's not so trivial to make voids of time by accident
* Lots of crash fixes
* yapf formatting, with tabs
* Improved documentation

* Fix an intermittent error where entity deletion sometimes wouldn't go through all the way
* Performance improvements
* LiSE now saves keyframes on close by default. Change ``keyframe_on_close`` to ``False`` to disable this
* Renamed "modulus" options to end with "interval" instead
* Disable the cache arranger by default

* ELiDE now has the Timestream view, easing time travel when history gets complicated
* Rename "avatar" to "unit"
* Nodes now support the `.user.only` property, just like characters' `.unit.only`
* Improve interprocess communication some more, particularly when time travelling between branches
* Tab indentation, with yapf formatting
* Rename some Engine methods to use snake_case
* numpy is no longer required for the LiSE core (will speed up IPC, though)
* Fix a loading error with changes in branches other than trunk
* Crash fixes
* Drop support for Python 3.6