SubDofHandler/MixedGrid/PorousMedia example

[KnutAM]

Closes #355

Example for using both mixed grid and fields on subsets of the domain: Porous media with embedded impermeable particles. Link to example in docs preview

[codecov-commenter]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (3ffb34f) 92.75% compared to head (9dc7ff4) 92.77%.
Report is 2 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #448      +/-   ##
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+ Coverage   92.75%   92.77%   +0.02%     
==========================================
  Files          33       33              
  Lines        4952     4952              
==========================================
+ Hits         4593     4594       +1     
+ Misses        359      358       -1     

see 1 file with indirect coverage changes

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[KnutAM]

This is my naive attempt for an example that I would have liked to have when starting using the MixedDofHandler
Probably a few blunders, so I would be happy to get your opinions @kimauth, @lijas, and @fredrikekre, on if this is reasonable and what should be changed in the implementation. Then I can spend more time on beautifying and comment/theory clarifications!
(The loading case should change to highlight more that it is a coupled problem, but that shouldn't affect the code)

[KnutAM]

Edit: Better to be explicit, improved theory included already.

Note for later, if to be included in Ferrite.jl:
A simpler and shorter theory section, different boundary conditions, and using AD for the stiffness, can be found here. These changes can be made here as well.

[termi-official]

Is there something blocking this PR from merge?

[KnutAM]

Is there something blocking this PR from merge?

Missing review from my perspective...

[KnutAM]

Looks like the increased CI doc time is due to updated packages after updating the docs project (need to upgrade FerriteMeshParser to 0.1.7 to support the master branch).