add a new class that will store the M, K and F.
add the content of this class.
switching on this class in one of the UserModules.
add activity model definition.
define the comon interface of activity model class.
add two classes handling Debye-Hueckel and Davies activity models
Unity activity coefficient model. All activity coefficients are ones.
small bug.
Implementation of re-using the transport operator finished.
Merge branch 'reuse_trans_operator' into nitrate_test
imposing an activity model on the equilibrium reaction system.