Warn when reading NaN as initial state or constant inputs
We should a warning when reading a NaN as a constant input or initial state. A quick glance tells me that the current state is that:
- PIMixin raises an Exception on NaN in timeseries, not even a warning in history/initial state
- CSVMixin does not do any NaN checks
Simulation: no checks
This behavior was caught by ed4412c3 , because the optimization run would run just fine but the simulation run would fail on the NaN.