`scale_factor` attribute to Configuration

[matteo-pacher]

When serializing a Configuration, I think it is meaningful to keep track of its scale_factor.
For example, if I plan a path using a scaled robot (robot.scale(1000)), the configuration's prismatic or planar joint values will also be scaled, and serialized as such. When deserializing that information, the scale_factor information is lost.

EPIC: @gonzalocasas mentioned the idea of a compas_units library that would take care of such issue.

[yck011522]

I testify it would be beneficial to my project workflow if the units are explicitly saved. Expecially because I serialize my data multiple times passing them between comaps_fab planning, visualization with Rhino and execution with compas_rrc.