Move anisotropic viscosity assemblers into main code

[gassmoeller]

In #2139 we removed the code for anisotropic viscosity tensors from the main code and moved it into separate assemblers in a test case and a cookbook. However, this lead to the duplication of the code, and #6615 now introduces another duplicate copy, which is of course undesirable. I think the correct solution would be to move the assemblers back into the main code, but as separate assemblers (to avoid the performance penalty mentioned in #2139). We do not need a special switch to replace the normal assemblers with the anisotropic viscosity assemblers at the moment, as so few models use them. We can keep using the current system of letting user plugins replace the existing assemblers with the anisotropic viscosity assemblers, the main issue is to get the classes back into the main codebase to make them available without duplication.