Use constructor and unroll function for tensors

[gassmoeller]

This PR is similar to #5640, except it only uses the existing unroll functions and constructors of the Tensor class to simplify the creation and unrolling of tensors from and into compositional fields.

[tjhei]

wouldn't be simpler to do

Suggested change

	const ArrayView<double> strain_increment_values(&out.reaction_terms[q][0],
	Tensor<2,dim>::n_independent_components);
	strain_increment.unroll(strain_increment_values.begin(), strain_increment_values.end());
	strain_increment.unroll(strain_increment.unroll(&out.reaction_terms[q][0], &out.reaction_terms[q][0]+Tensor<2,dim>::n_independent_components);

?

[tjhei]

I am also not sure why unroll actually needs an end iterator. The interface would be nicer if it would take a single iterator...

[bangerth]

I don't think this is easier to read. The issue with unroll() is that it isn't clear which direction the operation goes. Is the tensor putting itself into an array, or is it reading new values from the array? This was clear in the original code.

[gassmoeller]

I updated the PR according to the discussions in #5640. I cannot change the name of unroll here until I made the change in deal.II.

[bangerth]

Something is still broken -- the only thing that works is the spell checker :-)

[gassmoeller]

I guess that shows how much I tested this after the change :-). Fixed by deleting an unnecessary closing parenthesis.

[bangerth]

It's still not quite right:

/__w/aspect/aspect/tests/simple_shear_output_the_mobility.cc: In member function 'virtual void aspect::MaterialModel::FiniteStrain<dim>::evaluate(const aspect::MaterialModel::MaterialModelInputs<dim>&, aspect::MaterialModel::MaterialModelOutputs<dim>&) const':
/__w/aspect/aspect/tests/simple_shear_output_the_mobility.cc:85:35: error: redeclaration of 'const dealii::Tensor<2, dim, double> strain'
   85 |               const Tensor<2,dim> strain(strain_values)
      |                                   ^~~~~~
/__w/aspect/aspect/tests/simple_shear_output_the_mobility.cc:83:35: note: 'const dealii::Tensor<2, dim, double> strain' previously declared here
   83 |               const Tensor<2,dim> strain(make_array_view(&in.composition[q][0],
      |                                   ^~~~~~
/__w/aspect/aspect/tests/simple_shear_output_the_mobility.cc:85:42: error: 'strain_values' was not declared in this scope
   85 |               const Tensor<2,dim> strain(strain_values)
      |                                          ^~~~~~~~~~~~~

[gassmoeller]

Ok, third time's the charm. Seems to be working now.