Operator interface

@alexfikl brought up a great point about the current operator interface: https://github.com/inducer/grudge/pull/172#discussion_r729138306

As already pointed out, grudge operators typically have a function signature like: `operator(dcoll, *args)`, such as: https://github.com/inducer/grudge/blob/2af3528391bf4cb07f5aacf43f7c6afcc18f693a/grudge/op.py#L437

The issue here is that we're not quite enforcing anything on `*args`. So, for example, if a user passes in `(vec, dd)` (instead of the expected `(dd, vec)` ordering), the code will break without actually catching this problem. Currently, we have the docstrings elaborate on `*args`, such as: https://github.com/inducer/grudge/blob/2af3528391bf4cb07f5aacf43f7c6afcc18f693a/grudge/op.py#L438-L441

We could strengthen the checks inside each function. However, I can see this becoming a bit unwieldy. We could also revamp the interface and impose a fixed signature for each function (meaning we _always_ require args: `dcoll, dd, vec` --- no more `*args`).

I thought I'd raise this issue so we can discuss this. I don't think this is a huge problem right now, but definitely worth noting.