Unclear Documentation of noiseFormula

[FFroehlich]

Documentation of noiseFormula specified that its possible to specify numerical values here (conflicting with the type declaration as string), which doesn't seem to work (passing numerical values throws a mildly uninformative TypeError: expected string or bytes-like object error.)

Seems to be related to #264, but there is an issue with documentation nevertheless.

[dweindl]

Okay, 'string' in this case also included e.g. '1.0'. But should be clarified. TypeError should not have occurred.

[FFroehlich]

Okay after fixing other parts of the petab specification I no longer get TyperErrors and both numerical and string variables seem to work. Documentation may still need clarification though.