Housekeeping - remove outdated code

[TobyBoyne]

There are plenty of outdated things than need to be removed:

• Old tests that no longer work
• forest and forest_numba to be combined
• Move utils to more appropriate directories; remove utils.space
• Delete old alfalfa.fitting.bart code
• Rename project to bark

[TobyBoyne]

There is also some of the bark.optimizer code that isn't used, e.g. GbmModel.get_active_area

• Remove unused optimizer code