Be able to use datafunctions in parameter expressions

[ahellander]

On the Python modeling API level, URDMEDataFunctions should behave like Parameters, i.e. it should be possible to do something like

self.add_data_function(MeshSize(self.mesh))

And then say:

p = pyurdme.Parameter(name="width",expression="L/h")

(where L is some other parameter and h is the str name of the datafunction). The code generated in the C level would in principle work as is now, but p would get the value None in the C file since it is not evaluable only in the namespace of the parameter list.

Solution: evaluate Parameters in a namespace that is a union of data functions and parameters prior to serializing to file.