Refactor unit models

[tstorek]

Is your feature request related to a problem? Please describe.
Currently, filip is closely to the UNECE unit model. However, depending on the system requirements one want to add other unit standard,e.g., QUDT or BACNET.

Describe the solution you'd like

1. Refactor the unit-model to first become also independ from pandas as proposed by Can the installation of pandas/numpy be optimized/made optional #194
2. Integrate diffrent unit models depending on QUDT or BACnet and parse them based on their type definition, e.g. qudt:unit, bacnet:unit, unece:unit. We can make the default unit-type configurable via filip settings. I would stick to the unece as default as this is what the fiware community proposes. @SBlechmann is this correct=
3. Prepare a way to define CustomUnits, e.g. by defining an import path using filip settings

Describe alternatives you've considered
Stay with the current

Additional context
For future work this might be kind of relevant, e.g. when integrating with frameworks as https://pint.readthedocs.io/en/stable/

Also related to #186

[github-actions]

Branch 280-Refactor-unit-models created!

[tstorek]

@djs0109 any further suggestions?