Clean up wrapping of getter functions

[jwallwork23]

The getter functions such as

def get_form(mesh_seq):
    def form(index, solutions):
        ...
        return {field: F}
    return form

are cumbersome. We should clean things up. Some possible approaches:

• Decorators / wrappers
• Just add mesh_seq as the first argument of the function being returned, so that it can be easily converted into a method.