More clever options handling

We currently have several scenarios, where we want to generate multiple LocalOperators from one ufl file. With the current way, this is a mess. @rene.hess and me want to have something like this in the ini file:

[formcompiler]
operators = op1, op2

[formcompiler.op1]
form = r  # takes the object r from the uflfile
classname = LocalOperator  # name of the generated C++ class
filename = mysuperlocalop.hh  # name of the generated header
...
[formcompiler.op2]
form = mass
classname = MassOperator
...

@marcel.koch Does that sound good to you as well? Do you have something on your wishlist as well regarding options handling?