Blender Git Loki

Tests: added simple unittests for matrix interpolation

The interpolation of 4x4 and 3x3 matrices will fail when the rotation
component has a singularity, i.e. when there is one axis mirrored. Two
mirrored axes are just a rotation of 180 degrees around the third, and
three mirrored axes are such a rotation + a single axis mirror. To
prepare for a fix, I first wanted to cover the basic functionality with
a few unit tests.

These tests check that `interpolate(A, B, alpha)` always returns `A` for
`alpha=0`, always return `B` for `alpha=1`, and something in between for
`alpha=0.5`.