Make concerted reactions only allowed for a consistent reaction center

[samblau]

Concerted reactions only really make sense when the bonds being formed/broken are all connected to the same reaction center. This can also help with unphysical "robbing Peter to pay Paul" type concerteds where one reaction which is very exergonic can be unphysically combined with an unrelated endergonic reaction in order to facilitate a very uphill reaction which in reality will not occur.