Blender Git Loki

Add more tests to bpyunits.

Only LENGTH type for now.

This showed two limits to current BKE's units code:
* When converting string to value, we use too few precision for units' values (6 digits),
not good esp. when mixing big and small units (like e.g. miles and inches). Will fix that
in next commit.
* When converting values to string, 'precision' parameter ignores order of magnitude of the value,
which leads to invalid results with big numbers (e.g. 1000000mi gives 1000000.004mi with prec of 3).