Quadrature should define an exact_on_spaces attribute

The [current definition](https://github.com/inducer/modepy/blob/4b92c807bf7c65a0693735b9da2207a18374f125/modepy/quadrature/__init__.py#L67-L71) of `exact_to` is pretty narrowly restricted to P^N polynomials. Using spaces could let us express which spaces are exactly integrated. Exact on a set of sines and cosines? No problem! :)