Add substitution exchanges in activities

[cardosan]

While it is currently possible to have substitution exchanges in AB implicitly (i.e. flipping the sign of the technosphere exchanges), this makes the work confused and unintuitive.

Since brightway allows to deal with substitution exchanges explicitly, I would suggest to include in the Activity Details tab, other than technosphere and biosphere, also the substitution exchanges

[StpdFox]

Seems like a good idea , i'll bring it up in the next dev meeting!