FEValues: explicitly instantiate functions

[peterrum]

needed by #12756 (to be able to define FEValuesBase derived classes in other files)

FYI @simonsticko

[simonsticko]

This is more directly related to #12024 than to #12756, since we agreed to move the new class in #12024 to NonMatching.

To be able to use this in #12024 we also need a new header fe/fe_values.templates.h and then move these classes to it:
FEValuesBase<dim, spacedim>::CellIteratorBase
FEValuesBase<dim, spacedim>::CellIterator
Right? Not sure if that should go in this pull-request or a separate one.

[peterrum]

To be able to use this in #12024 we also need a new header fe/fe_values.templates.h and then move these classes to it:
FEValuesBase<dim, spacedim>::CellIteratorBase
FEValuesBase<dim, spacedim>::CellIterator
Right? Not sure if that should go in this pull-request or a separate one.

Exactly. I would do that in a follow-up PR. The PRs are easier to review this way.

[bangerth]

This is not maintainable. We can't just import hundreds of lines of auto-generated code. That's particularly true given that the origin of these lines is from an expansion of the type

for (VEC : VECTOR_TYPES)

where VECTOR_TYPES depends on cmake configuration settings. You'll have to find a different way of doing what you need.