Add radiation (#893)

* tweak docs

* cosmetic change

* add IsothermalSlipWallBoundary

* move project_from_base to utils

* allow wall temp to be non-constant in isothermal BCs

* convert to DOFDesc

* add radiation

* Add test for radiation

* set visualize=False by default

* epsilon -> emissivity

* one more epsilon -> emissivity in tests

* ValueError -> TypeError

* cosmetic change

* remove loop over mesh size from radiation test

not necessary since we aren't checking accuracy

* remove radiation from deprecated operators

* add missing radiation argument check to basic_coupled_ns_heat_operator

* don't return wall_grad_temperature from diffusion_operator since we already have it

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Co-authored-by: Tulio <tulioricci@gmail.com>