Plotter variables are not constants

Actually the ExaHyPE runtime treats the number of writtenUnknowns per plotter as a runtime variable, ie.

    plot hdf5::flash ConservedWriter
      variables = 19
      time = 0.0
      repeat = 0.00001
      output = ./hdf5-flash/conserved
    end plot

one can change variables = x without recompiling. However, the toolkit wants this to be a constant:

    plot hdf5::flash ConservedWriter
      variables const = 19
      time = 0.0
      repeat = 0.00001
      output = ./hdf5-flash/conserved
    end plot

otherwise it says ERROR: eu.exahype.parser.ParserException: [71,17] expecting: 'const'

This should not happen, ie. the toolkit grammar should accept without const.

As always, there is assumably no case when somebody wants to do this except benchmarking plotter file formats which is exactly what I'm doing now and the typical generated code by the toolkit is not aware of a non-constexpr number of variables, but all the ExaHyPE/exahype/plotters/ code actually treats the number as runtime constant and I see no reason why to artificially introduce something constexpr here.