Improvements for V2.0

[dgarijo]

I am adding here some of the contents of https://github.com/TEC-Toolkit/Emission-Conversion-Factors-Ontology/discussions/97 because I can never find it. A summary:

• Source and target should include an intermediate class that has the unit, instead of having source element, source unit.
• We have tags, but they refer to activities. For example "Butane" in reality refers to the activity "Burning Butane", which uses "Butane" as input and produces CO2 as a target. The current modeling is not wrong, it simplifies things. But this characterization seems more "correct".

When we integrate other sources, we will see whether we need to push for this change further.

[dgarijo]

Also please bear in mind that these changes have major impact in the mappings, calculator, etc.

[m-markovic]

We should also consider lining mentions of different Tier methods and uncertainty values as per the IPCC guidelines https://www.ipcc-nggip.iges.or.jp/public/2006gl/

• if available, add link between CF and the corresponding IPCC tier method
• if available, add link to uncertainty values associated with the specific CF