Merge branch 'feature/simple-diffusion' into 'master'

[example] Add simple diffusion eqution including full model

See merge request !3380

(cherry picked from commit 8a701244)

f334d2ef [common] Add BasicVolumeVariables for reuse in simple models
74f3d6ab [example] Add simple diffusion eqution including full model
369504e8 [doc examples simple-diffusion] first steps for docu of simple-diffusion
ddc1161a [example][diffusion][doc] Add documentation
43ed102f [example][diffusion] Test the diffusion example
5164f6f2 [example][diffusion] Document the BaseResidual property
3a9ad1fe [example][diffusion] Fix typos
1f67777d [example][diffusion] Improvements and corrections of the example