define URI's for "elementary flows" in exiobase

[mfastudillo]

As far as I understand we do not have yet URI's for "emissions" and other types of what in LCA is known as "elementary flows". In the case of bonsai I guess they are just bont:ActivityType. These are in several tabs of the "satellite accounts" of exiobase.

it is also unclear to me how the "compartment" / context is represented with the BONSAI ontology. e.g. PM2.5 emitted to air urban vs air rural. This needs to be specified to do the conexion with characterisation factors.

also linked to this issue