Noise formulas should not contain spaces

[FFroehlich]

specifying 'noiseFormula' as 'lambda reg' will make

https://github.com/ICB-DCM/AMICI/blob/16e3d762a94a35d21825f904d9d4381f82d117ab/python/amici/petab_import.py#L502

fail since sp.sympify('lambda reg'). I assume this will be a general issue and is not related to amici and that amici does sufficiently input check the provided petab.

[JanHasenauer]

Is this implemented in the validation? if not, this would be important.

[dweindl]

I think this has been adressed by #579