mainly: added diffusion boundary flux. The boundary treatment needs some
overhaul

needs all components to be limited.

hasDiffusion is true.

tried to fix the maxSpeed method and the way the face length should be used

a 'solve(target') method to the operator so that u_h_n is not required
anymore in the python script

- add enums to Additional
- make methods in models also depend on time - doesn't
  quite work yet because the 'time' is not correctly set
  on the 'DiffusionModel' (can't be since this is a virtual model wrapper)
  Need to find better way, i.e., setting it on python side or avoid virtual
  wrapper

- 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