Make boolean engine selection more flexible

[onitake]

Closes: #2306

As announced, here's a patch that refactors the boolean engine autodetection.

I had to refactor a few things to make the two engines behave the same way and avoid circular imports.

reduce_cascade is now in util.py. I assumed that this is needed in other places, so I didn't move it into the new interfaces.manifold module.

interfaces.blender and interfaces.manifold are now dynamic imports. This avoids a circular import loop and allows autodetection at load time.

[mikedh]

Awesome thanks for the PR!! Few quick thoughts:

• is there any reason we couldn't use trimesh.util.has_module for the autodetection? It seems pretty much the same idea as here.
• I've been meaning to move trimesh.boolean.reduce_cascade and trimesh.util.chain into their own trimesh.iteration submodule for the "materially similar to functions in itertools" stuff. I'll do that on merge which should reduce circular import problems.

Is there a side effect of try: import manifold3d? Because we could also pick the default by using the first one that isn't an ExceptionWrapper:

# which backend boolean engines                                                            
_engines = {
    "manifold": boolean_manifold,
    "blender": interfaces.blender.boolean,
    "junk": ExceptionWrapper(ValueError("nah")),
}
_engines[None] = next(
    (v for v in _engines.values() if not isinstance(v, ExceptionWrapper)), None
)

[onitake]

Looks like it didn't quite work as well as I had hoped.

Thanks for the pointers, I'll take a look at has_module.

[mikedh]

No worries! I'll play around with it in my branch a bit. Thanks for the PR!!