Exploit diagonalness of the mass matrix when applying tensor product mass inverse

When?
- If using (tensor product) Gauss-Legendre points,
- or if using Lobatto-Gauss-Legendre points with permission to lump (#348).

This is related to #349, but for the inverse operator.

Sample reference: https://doi.org/10.1002/fld.3923

cc @MTCam @a-alveyblanc @lukeolson