Adding shear heating operator

This is to mainly add the shear heating term for TALA which is needed for the TerraNeo app hyteg#243 (closed), which is very similar to the Stokes form but still cannot be readily used in the energy equation as the function spaces are different.

I did a simple integration test (math) for the IdentityMap which looks fine. Have also added the literature references in the comments of the form for cross checking.