Enable diff mode between data sets

[davidscn]

We have the so-called 'diff'-mode in precice-aste-evaluate in order to compute error metrics on our output mesh. The diff mode takes as input arguments the diffdata (data used for the difference calculation) and a function in order to generate reference data. However, computing the difference between two data fields is not possible. We should extend the current CLI to also accept a second ('reference') data field and compute the error between these two data fields instead of an analytic function.

[davidscn]

A bit of an open question is, how the user interface then looks like. Currently, we have

The --diff flag calculates the difference between "--diffdata" and the specifiedfunction "--function"

precice-aste-evaluate -m map_nn.vtk -f "franke3d" -d difference --diffdata "InterpolatedData" --diff

Here, the diffdata is already the data we use to compute the difference and difference is the name of the data we store on the mesh file. We could make the function argument optional and enable two 'diffdata':

precice-aste-evaluate -m map_nn.vtk -d difference --diffdata "InterpolatedData" "ReferenceData" --diff

[uekerman]

Alternative could be an optional --refData?