Convergence test

A convergence test macro (protype implemented in merge request !6 (closed)) allows to add a convergence test (possibly in time or space) running the same executable several times with different ini files.

In the future the programme has to output the convergence error and hmax as a Dune::ParameterTree. A python script can process this output, calculate rates and fuzzy compare them to an expected rate specified in the metaini file. Possible syntax here:

[__CONVERGENCE_TEST]
Variable = pressure
NormType = L2
ExpectedRate = 2
AbsDiff = 0.1
TestKey = Grid.Level # in case we also allow dynamic variations