Fix show of Face and Cell values

[KnutAM]

For upcoming 1.0, make sure show doesn't fail for face values (closes #752)
Makes output for CellValues consistent with FaceValues.

Note: Exact output may change later (e.g. if/when we split #764 or similar), this is just to remove bug for the upcoming release.

Output:

# Scalar
CellValues with
- Quadrature rule with 4 points
- Function interpolation: Lagrange{RefQuadrilateral, 2}()
- Geometric interpolation: Lagrange{RefQuadrilateral, 1}()

# Vector 
CellValues with
- Quadrature rule with 4 points
- Function interpolation: Lagrange{RefQuadrilateral, 2}()^2
- Geometric interpolation: Lagrange{RefQuadrilateral, 1}()

FaceValues with
- Quadrature rule with 2 points per face
- Function interpolation: Lagrange{RefQuadrilateral, 2}()
- Geometric interpolation: Lagrange{RefQuadrilateral, 1}()

# Different number of quadrature points on each face:
FaceValues with
- Quadrature rule with (3, 2, 2, 2) points on each face
- Function interpolation: Lagrange{RefQuadrilateral, 2}()
- Geometric interpolation: Lagrange{RefQuadrilateral, 1}()

[termi-official]

Maybe the smoke test should be with a prism or pyramid to capture possible regression here. Otherwise really nice fix!

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.49% 🎉

Comparison is base (a014185) 91.77% compared to head (7cb52a0) 92.27%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #771      +/-   ##
==========================================
+ Coverage   91.77%   92.27%   +0.49%     
==========================================
  Files          32       32              
  Lines        4720     4735      +15     
==========================================
+ Hits         4332     4369      +37     
+ Misses        388      366      -22     

Files Changed	Coverage Δ
src/FEValues/cell_values.jl	100.00% <100.00%> (ø)
src/FEValues/face_values.jl	98.27% <100.00%> (+0.14%)	⬆️

... and 2 files with indirect coverage changes

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[KnutAM]

Maybe the smoke test should be with a prism or pyramid to capture possible regression here

Added for CellValues only since ERROR: FaceQuadratureRule for RefPrism not implemented (#772)