Move unit conversions to separate file

Unit conversions shouldn't be in the same file as physical entities, such as e.g. the speed of light (see #690 (closed)). Instead move them to a new file and a new namespace.

changed milestone to %2022.1

added Cleanup label

mentioned in merge request !552 (merged)

Closed with 066ad356

closed

marked this issue as related to #357

mentioned in issue #227

marked this issue as related to #690 (closed)