Implement and test TFP

[ruestefa]

The computation of the frontal velocity used in the front identifcation is based on the direction of the gradient of THE, not on that of TFP as previously thought (and as documented in the thesis and paper), the reason being that the Fortran front tool used is that adapted to COSMO-2, not the original one used in Jenkner et al. and for ERA-Interim.

• Implement TFP as an optional alternative to THE in the computation of fvel.
• Compare the two implementations in a few test cases (winter and summer).