Optimizer crash if SDDSVariable return value is NaN.

I recently observed that the optimizer crashes with

Error{0}> 
Error{0}> *** Error:
Error{0}>     Internal OPAL error: 
Error{0}>     input stream error
Error{0}>     input stream error
Rank 0 [Wed Jun 26 20:01:25 2019] [c0-0c0s2n0] application called MPI_Abort(MPI_COMM_WORLD, -100) - process 0
SIGABRT

if the return value of SDDSVariable is ǸaN. With a check std::isnan and std::isinf this can be fixed. It's probably best it's added to the SDDSParser which then throws an execption that is caught by SDDSVariable.