Add provenance of numerical value of a Flow as a mathematical formula with variables

Add the possibility to document how the numerical value of a Flow has been calculated (when not being a direct measurement). The minimum would be to give the formula as a provenance information (already opened as a separate [issue](https://github.com/BONSAMURAIS/BONSAI-ontology-RDF-framework/issues/5)). A more advanced and preferable solution would be something corresponding to the @mathematicalRelation of the [ecoSpold2 format](https://www.ecoinvent.org/data-provider/data-provider-toolkit/ecospold2/ecospold2.html) that defines a mathematical formula which can also contain variables and which will fill the value of the flow if ecospold2:@isCalculatedAmount is TRUE. This is a very explicit and recommendable option for providing provenance information. In ecoSpold2 the formula are defined by a sub-set of the OpenFormula standard. Other RDF-related formula standards are described on the Wikipedia-page for MathML. It should be noted that uncertainty for a calculated number is calculated from the uncertainty of the contributing numbers plus any uncertainty on the mathematical relation itself.