Better format for input to Polygon_function

[ole]

The curren format is confusing: A list of tuples of the form (P, v) where P is a polygon which in turn is a list of points and v is a value or a function.

We need some sort of object model.

class Functional_region:

# This contains a Polygon and a value (constant, function, dataset, ....)

and then the Polygon function takes a list of these instances as input.