Updated the Python interface of the material.

The material is parsable, and as such it offers the functions set() and get() to change and retrieve internal variables.
However, when the parameter changes some internal values might need to be recomputed, which is done by Material::updateInternalParameters() function.
But the set() function does not do this.
For that reason the C++ interface of the material offers the setParam() function, which basically does this.

This commit now modifies the Python interface of the material and redirects the set*() functions, that are defined with by the parameter registry, to the setParam() function.
Thus, if a material property is changed by a call to set*() the internal values are recomputed.