Idea: Enable a couple of clang-tidy checks

[phako]

Especially readability-magic-numbers seems quite useful.

• Review checks at https://clang.llvm.org/extra/clang-tidy/checks/list.html
• See how to hook that into Jenkins

[mhier]

+1

But readability-magic-numbers will constantly annoy us e.g. in tests where magic numbers are perfectly fine. Also in formulae we quite often have "magic numbers" like 2 (which is already not allowed by default). This one particularly may be just too strict.