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

be sure