Introduce an interface class between the UFL visitor and the PDELab generator functions

Thinking about the design of the sum factorization backend I come to the conclusion that this is a good idea.

Current state:

class UFL2LoopyVisitor:
    def argument(self, o):
        ....
        return .... name_basis() ...
        ....
   ... many more handlers calling generator functions

Proposed solution:

class UFL2LoopyVisitor:
    def argument(self, o):
        ....
        return .... self.interface.name_basis() ...
        ....
   ... many more handlers calling generator functions

class PDELabInterface:
     def name_basis(..):
           return name_basis(...)
     ... many more generator functions ....

That would mean, you can inherit a SumFactPDELabInterface from PDELabInterface and feed an instance of it into the visitor instead. This might sound like it is contradicting the previously implemented backend-selection mechanism, but it is actually supporting it for those switches, where basically everything changes.