Checking constraints on parameters in moduli functions

[akabla]

We have some tests as part of the expressions themselves in the model declarations, and they are not always consistent with the constraint expressions. (Discussion stared as part of #111)

I suggest to remove the tests in the expressions of the moduli (unless that would cause terrible issues with other parts like NLopt) and rather make sure that we test parameters in functions such as relaxmod, creepcomp, etc., throwing an exception with explicit error message.

This is also related to #86.

The issue of the specilisation of models for certain parameters values might be worth considering here too. If a Springpot with exponent 0 is considered, should Rheos know that this is a Spring and use the specialised model instead?