Higher order gradients in CellValues

[lijas]

Implementation of higher order gradients in CellValues. This was quite easy to do thanks to Knuts new FunctionValues and GeometryMapping.

• [x] Tests for FaceValues
• [ ] Update changelog
• [ ] Type stable CellValues constructor (?)

[KnutAM]

Type stable CellValues constructor

Merging with master and using the same concept for update_hessian as for update_gradient should solve this problem.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 92.24138% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 93.12%. Comparing base (8e92196) to head (f16e2a5). Report is 1 commits behind head on master.

Files	Patch %	Lines
src/FEValues/FunctionValues.jl	87.27%	7 Missing :warning:
src/interpolations.jl	92.30%	2 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #891      +/-   ##
==========================================
- Coverage   93.30%   93.12%   -0.18%     
==========================================
  Files          36       36              
  Lines        5257     5384     +127     
==========================================
+ Hits         4905     5014     +109     
- Misses        352      370      +18     

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