there seems to be only a *very* limited amount of interest, the code
of the model is quite complex and there are currently no suitable
discretizations for free-flow equations in eWoms (i.e., the model
tends to be very unstable and oscillates a lot). Combined, all of this
makes maintaining this model a pain in the back, so let's remove it
some interest in these kinds of problems surfaces and until
appropriate discretizations -- like staggered grid methods -- are
available.