Weighting options for Difference Energies

To aid interpretation the difference energies they can be weighted by mass, or volume. This issue addresses the addition of the weighting function to all difference energies (DKE, DTE, Temperature variance).

Start by locating the implementations and tests for the difference energies named in the issue: DKE, DTE, and temperature variance. Review how mass or volume weighting is represented elsewhere, then verify that the weighting function is applied consistently to all three energies and that their tests cover the new behavior.

Written by the indexing model from the issue text.