child_mappings for root

[suleman-uzair]

There should be child_mappings like functionality for root and named accordingly(collection_mappings or root_mappings).

Since all the .yaml files in unitsml/unitsdb (except unit_systems.yaml) contains dynamic root names.

For example NISTp10_30 and NISTp10_27 in prefixes.yaml and "NISTu1" and "NISTu7" in units.yaml.
The child_mappings functionality at the root level can be useful in this scenario.