Add tree tick_mode

[Flynsarmy]

Description

This PR adds a tick_mode option to BeehaveTree with the following options:

• BOTH - ticks on _process and _physics_process
• PROCESS - ticks on _process
• PHYSICS - ticks on _physics_process
• MANUAL - doesn't tick at all. Call $YourTree.tick() to tick the tree.

This PR is backwards compatible as it defaults to BOTH which is the existing functionality. Existing trees will continue working exactly as they always have.

There are a couple of reasons for this change:

• I want to be able to tick some trees at my own custom rate. The tick_rate option kind of accomplishes this but setting to MANUAL offers the ultimate in flexibility and let's the developer decide exactly how and when the tree ticks
• The current default of BOTH is kind of dangerous and your unit tests don't cover this. _process runs an unknown number of times per second so tick_rate isn't reliable.

Fixes #389