WIP: Fitch wind farm parametrization

[nataraj2]

This PR introduces the Fitch parametrization for wind farms
Fitch model 2012
This parametrization for wind farms involves adding source terms to update the x and y velocities as well the turbulent kinetic energy in the mesh cells where the wind turbines are located. Source/WindFarmParametrization/Fitch contains the Fitch model source terms. The paramterization is a runtime option - windfarm_type. Additional parametrizations will be added later.

An example is provided in Exec/Fitch. The turbine locations are in the attached figure and the movie shows the evolution of the velocity magnitude at the hub height of the turbines of z=100 m. All other features such as dissipation and turbulence have been turned off in inputs_Fitch to demonstrate the feature.

To do:

1. Read in the parameters from the WRF tbl file.
2. Introduce drag coefficient for the turbines as in WRF.
   
   

[asalmgren]

in fitch_update it would be good to put all those loops into a single ParallelFor (over multiple boxes).

[asalmgren]

Also -- we will want to implement other models that do similar things to Fitch but are somewhat different models -- do you want to go ahead and put that generality in now so the choice of parametrization can be a run-time parameter?