Implement isFixed() method in various objects, to check whether
        they depend only on fixed parameters.

This is necessary for pdfs and pdf expressions to decide whether to
        cache values, since it depends on whether these expressions
        depend on fixed or released parameters.
Make the dataset size a size_t variable, instead of int.
Implement caching of pdf values.
Correct friendship of operators involving Parameter objects.
Add the abilities to get and set the parameters of a CoefExpr and to
        evaluate it.
Add two methods to allow to fix or release a parameter by its name.
Add new class Decay3BodyCP, which takes an amplitude A, a coefficient
        expression z, and a parameter k, and evaluates to
        |A|^2 + |z|^2 |Abar|^2 + 2 k Re( z A* Abar )
        at each given point.

It uses all the currently implemented caching techniques.