speciesSorption is a zeroGradient BC which absorbs mass given by a first
order time derivative, absoprtion rate and an equilibrium value
calculated based on internal species values next to the wall.
patchCellsSource is a source fvOption which applies to the corresponding
species and apply the source calculated on the speciesSorption BC.
A new abstract virtual class was created to group BC's which
don't introduce a source to the matrix (i.e zeroGradient) but calculate
a mass sink/source which should be introduced into the matrix. This
is done through the fvOption patchCellsSource.
