Make KE functional

[charleskawczynski]

This PR changes kinetic_energy! to be functional.

Some advantages to this form:

• It's easier to test (we can test it for a single point instead of a full field)
• It's easier to fuse because the function kinetic_energy! is effectively manually inlined
• It uses less parameter memory. This is because the broadcast expression only has one variable per used variable. For example, before we had local_geometry_halflevel that occurred in multiple places.