Blender Git Loki

no need to use mul_vn_fl when size is known

minor edits to expmap functions

add quat_normalized_to_expmap incase we know the quats already unit length

mathutils: added exponential map to Quaternion

Added conversion to and from exponential map representation. This representation is useful for interpolation of > 2 quaternions, or in PD controllers.

Implementation in C functions `quat_to_expmap(...)` and `expmap_to_quat(...)` with Python API, unit tests and documentation.
Added Quaternion.to_exponential_map() and Quaternion(3-vector) to Python API.

Reviewers: campbellbarton

Projects: #bf_blender

Differential Revision: https://developer.blender.org/D1049