Add configuration option to select/filter output variables

[ghorwin]

Allow selecting of output variables to write (as filter, since currently all are written).

This is really a convenience issue and only needed in the (rare) case, that one needs to log internal variables and a (Modelica) model just has very very many internal output variables that will be dumped alltogether. In most cases there is no significant performance penalty in writing a few more bytes to output.