`scale_factor` attribute to Configuration

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.