Operations on heterogenous matrices

I'm implementing an algorithm right now that operates on bilinear groups, and the `pairing` crate splits these into five different types of numbers.

The following operations are of importance to me:
- `G1Projective` + `G1Affine` → `G1Projective`
- `G2Projective` + `G2Affine` → `G2Projective`
- `pairing`(`G1Affine`, `G2Affine`) → `Gt`

I want the first two to be available component-wise (i.e. suppling a custom addition function to Matrix `+`) and the final one to be available as a multiplication operation (i.e. supplying a custom function to Matrix `*`). Is there a way I can do this?

Another (family of) operation(s) I ran into needing after submitting this issue: multiplying an `Matrix

Start by reading the Matrix + and Matrix * operator entry points and how nalgebra currently constrains element types. Check whether custom component-wise addition, pairing multiplication, and affine-by-scalar multiplication can fit the existing API, then verify that the requested G1Projective, G2Projective, and Gt operations are expressible and documented.

Written by the indexing model from the issue text.