Piecewise function outputs when outputting list of tendencies

[ushham]

When exporting the list of tendencies (in any language), the tendencies include the sympy Piecewise function due to the parameter n not being specified as positive=True. This is expected behaviour, but can make the outputted equations very messy and more difficult to translate into other languages.

[ushham]

I would suggest the following:

1. Either notify the user in the documentation that they should specify positive parameters explicitly.
2. Include an attribute in the Parameter class that allows the user to select positive, which then overwrites the behaviour of the sympy symbol that is passed.

[jodemaey]

For 1 we can give a hint in the documentation about that, but in the end the user decides the positivity (or other attributes) of his parameters.
2 is nice.