Generic cell-average output

• For TRM, LD, SD, TH2M processes.
• Provides cell-average output for all quantities using "reflection-based" IP data I/O.
• Averaged output of internal material state data (e.g. of MFront models) not yet implemented.
• The implementation is tested with a unit test (which might be a bit too similar to the actual implementation of the feature).
• I also added a unit test for raw/non-averaged IP data output.
• Using this feature in a new process is almost as easy as it can get. It requires only one line of code (zero would be better, but I don't know if that's possible to achieve):

    cell_average_data_.computeSecondaryVariable(DisplacementDim,
                                                local_assemblers_);

Update I've added a series of commits at the end. There I split the cell averaging into two files. The reason for this is to avoid that all reflection stuff is pulled in to Process.h, which makes incremental builds less effective. Therefore, now in each process the following code is necessary to use generic cell averaging:

#include "ProcessLib/Output/CellAverageAlgorithm.h"
...
    computeCellAverages(cell_average_data_, DisplacementDim, local_assemblers_);

1. Feature description was added to the changelog
2. Tests covering your feature were added?
3. Any new feature or behaviour change was documented?