Avoid missing symbols and use constexpr functions instead

[kronbichler]

Since #15137 has been merged, we have some linker errors, e.g. this one https://cdash.dealii.org/test/50357345 in the build https://cdash.dealii.org/viewTest.php?onlyfailed&buildid=4614 (look at the ones with point_evaluation). Technically, we would need to instantiate the static constexpr member variables. Since I do not want to do this for all types of VectorizedArray and Number we might have, my preference is to instead have constexpr functions similar to VectorizedArray::size(), which should avoid the problem altogether. Opinions?

FYI @bergbauer

[bergbauer]

Looks good. Can we now use width() in the Asserts in VectorizedArrayTrait<>::get()? I could not use it because of linker errors before.

[kronbichler]

Let's see if this works on the tester, otherwise I will look for some other solution.

[bangerth]

Do we care about the lack of backward compatibility?

[peterrum]

Do we care about the lack of backward compatibility?\

This is internal functionality.

[bangerth]

I believe that the failing test is unrelated.