- export a 'applyLimiter' method (needs to be implemented correctly for 'unlimited' operator
  Just for testing at the moment
- added 'x' dependency to some of the model methods.
  Need to discuss dependency on time as well

both the Model and the ufl expression for the initial data, i.e., only a
single function call needed to set up a test case. Later we can use run
time parameters to choose the problem

Also added static const ints for haveDiffusion,haveAdvection to
'Additional'

Added an advection/diffusion test case

be sure