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,
quat_normalized_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
Those two mimic our BLI invert_m4_m4_safe - they add a small offset to diagonal values,
in case org matrix is degenerated, and if still non-invertible, return identity matrix.
Org patch by me, final enhanced version by ideasman42, many thanks!
