These default tree definition options are non-obvious,
so although their use may slightly shorten code in a few situations,
the code would be more difficult to understand.
To check for backwards-compatibility problems,
`treecapitator.register_tree` now checks if these three options are set when required.
Furthermore, `treecapitator.register_tree` now no longer modifies the passed table in-place.
This is a breaking change since there might exist mod which uses the default values.
Drop support for minetest versions without minetest.override_item
Disallow registering trees before registering their trunk nodes
Abort if nodes can't be overridden correctly
Code style fixes
