DiscretizationCollection constructor should do no work

[This](https://github.com/inducer/grudge/blob/0a7b2fb2e4e170809b55e0c3c193ae1dbc4a61a4/grudge/discretization.py#L74-L162) is too fat. The constructor itself should just be an attribute setter. The actual work should be done by a "maker function", like `make_discretization_collection`. This is a long-term thing that'll hopefully make it easier to move towards moving geometry.

x-ref https://github.com/inducer/meshmode/issues/123