Visualization for 3D Heater Julia Example

[hammy4815]

Hi all. This PR contains my implementation for visualization of the 3D heater. I am using GridapMakie with some extra work around it to make it more readable and allow subplots.

So far, I've added a visualization for the mesh, and plots for the potential, current, temperature, and heat flux. I have also included animations for the transient simulation, but it takes awhile to compute so we may want to keep that part commented out.

Ideally, one would be able to interact with the plot since 3D visuals are nicer when interactive. However, I don't think this is possible in the docs export via Makie without binder or some other way to run a Julia environment live, but happy to be proven wrong here.