Improve assert message for non-matching reference cell types

[mschreter]

This PR suggest to improve the assert message if two reference cell types don't match, from

(old) 

An error occurred in line <2644> of file </scratch/lib/dealii/include/deal.II/dofs/dof_accessor.templates.h> in function
    const dealii::FiniteElement<dimension_, space_dimension_>& dealii::DoFCellAccessor<dim, spacedim, lda>::get_fe() const [with int dimension_ = 2; int space_dimension_ = 3; bool level_dof_access = false]
The violated condition was: 
    this->reference_cell() == fe.reference_cell()
Additional information: 
    The reference-cell type of the cell does not match the one of the
    finite element!

to

(new)
--------------------------------------------------------
An error occurred in line <2641> of file </home/magdalena/code/external_libs/dealii-work/include/deal.II/dofs/dof_accessor.templates.h> in function
    const dealii::FiniteElement<dimension_, space_dimension_>& dealii::DoFCellAccessor<dim, spacedim, lda>::get_fe() const [with int dimension_ = 2; int space_dimension_ = 3; bool level_dof_access = false]
The violated condition was: 
    this->reference_cell() == fe.reference_cell()
Additional information: 
    The reference-cell type of Tri does not match with Quad. This could
    happen if one object (e.g. Triangulation) holds reference cells from a hexahedral mesh and
    the other one (e.g. FiniteElement) from a simplex mesh.

This can happen if one attaches a simplex triangulation to the DoFHandler and subsequently calls DoFHandler::distribute_dofs(fe) with a FE type for hexahedral elements, e.g. FE_Q<dim>.

@tinhvo-TUM @peterrum FYI

[peterrum]

/rebuild

[bangerth]

Let's not invent a whole new exception class for just one place. This is more easily done in the following way:

Suggested change

	internal::ExcNonMatchingReferenceCellTypes(this->reference_cell(),
	fe.reference_cell()));
	ExcMessage ("The reference-cell type used on this cell (" + this->reference_cell().to_string() +
	") does not match the reference-cell type of the finite element associated with this cell (" +
	fe.reference_cell.to_string()
	+ "). Did you accidentally use simplex elements on hypercube meshes (or the other way around), or "
	"are you using a mixed mesh and assigned a simplex element to a hypercube cell "
	"(or the other way around) via the "
	"active_fe_index?"));

[peterrum]

I would suggest to keep the class, since, if I remember correctly, there are other places, where it could be used.

[mschreter]

By performing a quick search I could at least replace one additional assert message. I'll have a deeper look if there are more relevant places. Then we can decide.

[bangerth]

If you need a new class, use DeclExceptionMessage instead of doing it by hand, though.

[mschreter]

Done!

[bangerth]

I still think that this message is too specific. What do you think of the text I had in the other comment?

[mschreter]

Excellent, thanks, I like it and adopted it. I got so caught up in the other part that I didn't notice the changes in the text initially :-).

[bangerth]

Excellent, thank you!