Function blender_armature::blend_towards_bones

pub fn blend_towards_bones(
    start: &BTreeMap<u8, Bone>, 
    end: &BTreeMap<u8, Bone>, 
    interp_param: f32
) -> BTreeMap<u8, Bone>

Blend from the start bones towards the ending bones.

TODO: Delete. We now favor blending once at a time since this makes for a simpler API with fewer allocations