Bugfix/rans weightedaverages

When calculating the flux term in three of the rans models, a weighted arithmetic average was used. The cell size for each value was used as the weight. This means that the larger cell (with a cell center further from the face) will we weighted more than the smaller cell (with a cell center closer to the face). If we want to interpolate linearly, the weighting should be switched. With this change the convergence mildly improves.

In addition, a convenience function is added to the turbulencemodel.hh, and output placed in the problem. This will confirm in the output which turbulence model is being used.