How-to: L2Project flux onto H(div) space

[KnutAM]

Builds on #867

Needed to make some adjustments to L2Projector. Unfortunately, I couldn't find an automatic way to make it work in a backwards compatible fashion, since we rely on vectorized interpolations being de-vectorized. Probably possible, was just to tired.

For now, passing L2Projector{:scalar} is default and one can call L2Projector{:tensor} to directly use tensor-valued interpolations.

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Codecov Report

Attention: Patch coverage is 0% with 137 lines in your changes missing coverage. Please review.

Project coverage is 0.00%. Comparing base (6eead25) to head (a0c3a38).

Files with missing lines	Patch %	Lines
src/Export/vtk_discontinuous.jl	0.00%	58 Missing ⚠️
src/L2_projection.jl	0.00%	46 Missing ⚠️
src/Export/VTK.jl	0.00%	26 Missing ⚠️
src/Dofs/DofHandler.jl	0.00%	4 Missing ⚠️
src/interpolations.jl	0.00%	3 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (6eead25) and HEAD (a0c3a38). Click for more details.

HEAD has 5 uploads less than BASE

Flag	BASE (6eead25)	HEAD (a0c3a38)
	6	1

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #1161       +/-   ##
==========================================
- Coverage   93.74%   0.00%   -93.75%     
==========================================
  Files          39      40        +1     
  Lines        6238    6269       +31     
==========================================
- Hits         5848       0     -5848     
- Misses        390    6269     +5879     

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