Improve checks for existence of moduli/compliance expressions

[akabla]

This follows up from #52

Fields could be missing in the model definitions and the default value returned by the moduli/compliance functionwrappers is NaN, which seems appropriate.

• We may want to reconsider how the "modulusexists" function works though. This utility function could look into the "expressions" named tuple as part of the RheoModelClass object and determine then if the modulus is defined, rather than evaluating the function.

• processing.jl would need to be updated.

• The external moduli/compliance functions could also check and send an error where appropriate.