Option to set axis' manually on armature import

[TackYs]

When importing armatures, axis are currently set automatically by the guess_orientation function in nif_import/armature/init.py:

# use heuristics to determine a suitable orientation 
forward, up = self.guess_orientation(n_armature) 
# pass them to the matrix utility 
math.set_bone_orientation(forward, up)

Some times it would be valuable to be able to set these manually, example is if you have a rigged armature in the scene with a specific orientation and want to import another armature with matching orientation.

I'd love to give this a shot and pull request, is this a reasonable feature though?

Wrote this on mobile, hope formatting comes out ok.

[neomonkeus]

You would need to update the UI

• to have some form of override toggle.
• provide the user with the up, forward values.

Then update the function shown above to, before doing the guess check

• if the override is set,
• if set, set forward, up to the values provided by the user