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.

• 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.

[KnutAM]

Haven't checked all the details yet, but very nice to include this!

I think it would be nice to include the used formulas here, and also short derivation in a collapsed block as for the non-identity mappings.

[KnutAM]

Suggested change

	Jinv = calculate_Jinv(getjacobian(mapping_values))
	H = gethessian(mapping_values)
	@inbounds for j in 1:getnbasefunctions(funvals)
	dNdx = dothelper(funvals.dNdξ[j, q_point], Jinv)
	is_vector_valued = first(funvals.Nx) isa Vec
	if is_vector_valued #TODO - combine with helper function ?
	d2Ndx2 = (funvals.d2Ndξ2[j, q_point] - dNdx⋅H) ⊡ otimesu(Jinv,Jinv)
	else
	d2Ndx2 = Jinv'⋅(funvals.d2Ndξ2[j, q_point] - dNdx⋅H)⋅Jinv
	end
	Jinv = calculate_Jinv(getjacobian(mapping_values))
	H = gethessian(mapping_values)
	is_vector_valued = first(funvals.Nx) isa Vec
	Jinv_otimesu_Jinv = is_vector_valued ? otimesu(Jinv, Jinv) : nothing
	@inbounds for j in 1:getnbasefunctions(funvals)
	dNdx = dothelper(funvals.dNdξ[j, q_point], Jinv)
	if is_vector_valued #TODO - combine with helper function ?
	d2Ndx2 = (funvals.d2Ndξ2[j, q_point] - dNdx⋅H) ⊡ Jinv_otimesu_Jinv
	else
	d2Ndx2 = Jinv'⋅(funvals.d2Ndξ2[j, q_point] - dNdx⋅H)⋅Jinv
	end

[codecov]

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 ⚠️
src/interpolations.jl	92.30%	2 Missing ⚠️

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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