Remove deprecated method Fufem::istlVectorBackend

Instead, the method Dune::Functions::istlVectorBackend should be used.

added 12 commits

• 0928545f...8455462e - 6 commits from branch master
• 10de2ca7 - Remove dunefunctionsassemblertest
• 21acfe55 - Canonicalize a few single-inclusion macros
• 81d94bd5 - Remove unnecessary std::move
• b3b005ad - Remove deprecated method Fufem::istlVectorBackend
• dd6bc7f1 - Remove deprecated class MassAssembler
• ca263246 - Port test away from old LaplaceAssembler

Compare with previous version

added 8 commits

• ca263246...194b1aa3 - 2 commits from branch master
• bbf92983 - Remove dunefunctionsassemblertest
• dc2be071 - Canonicalize a few single-inclusion macros
• 8cb9b9aa - Remove unnecessary std::move
• 321511b0 - Remove deprecated method Fufem::istlVectorBackend
• 75a34f98 - Remove deprecated class MassAssembler
• e2e85f0f - Port test away from old LaplaceAssembler

Compare with previous version

merged

mentioned in commit c9754fd1