CellMultiValues (new attempt)

[KnutAM]

Supersedes #680 and #674, using the components from #764

• Check performance
• Update incompressible elasticity example
• Add tests
• Complete documentation

A couple of notes on the design

• Given a cv::CellMultiValues, cv[:u]::FunctionValues, which can be used just like a CellValues for all function-related queries, but not geometrical such as getdetJdV, these are called on cv itself.
• The constructor accepts a named tuple of function interpolations. If two interpolations are identical, they will share the same FunctionValues object, saving both memory (bandwidth) and computations in reinit!.
• FunctionValues subtypes AbstractValues to make all function_value and similar work. This implies that e.g. spatial_coordinate(cv[:u]::FunctionValues, args...) will give a method error on getngeobasefunctions instead of spatial_coordinate. We could add specializations if easier error messages are required.

It would be very convenient to also store the dof_range of each field, but having this as an input would be error-prone if the user provides the wrong order compared to what is stored in the SubDofHandler. If a convenience constructor similar to #806 would be used, this would be safe, but that should only be a high-level constructor interface. Ideas are welcome! (Edit: not generally applicable, so leave out for now)

[codecov]

Codecov Report

Attention: Patch coverage is 98.07692% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 93.33%. Comparing base (58ca229) to head (8016860).

Files	Patch %	Lines
src/FEValues/CellValues.jl	98.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #872      +/-   ##
==========================================
+ Coverage   93.30%   93.33%   +0.02%     
==========================================
  Files          36       36              
  Lines        5257     5308      +51     
==========================================
+ Hits         4905     4954      +49     
- Misses        352      354       +2     

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[KnutAM]

I've renamed to CellMultiValues following @termi-official's suggestion. I find this name harder when coding, but that could be me having become accustomed to MultiCellValues.
Another option could be CellValuesGroup

Otherwise, ready for review from my side if anyone wants to take a look!

[termi-official]

I left some comments.

Since I am struggling recently with the question: Do you think this should be still Ferrite core or in some package on top?

[KnutAM]

Do you think this should be still Ferrite core or in some package on top?

I think this belongs in Ferrite core for two main reasons:

1. Using multiple fields is common, and this feature makes that efficient and easier.
2. Implementing this without heavily relying on Ferrite internals is hard for an external package.

But of course, this is my biased opinion :)