Support quantity groups, units and conversions with a unit ontology database.

[francoislerall]

For example, these two organisations provide various quantity and unit standards:

• https://qudt.org/
• http://www.ontology-of-units-of-measure.org/

Two possibilities:

• create an action in openLCA to retrieve data and update units and conversion,
• automatically update units and conversion when creating a new release.

[msrocka]

Closing this issue here because we can use the data repository for such issues/fixes. When creating the reference database templates, it fetches the data from this repository now.