Revert "Revert "feat(timeit): add function timing decorator (#118)""

[wpbonelli]

• reverts Revert "feat(timeit): add function timing decorator (#118)" #119, pardon the indecision
• seems justified if devtools is used in notebooks as proposed in Self-contained example notebooks modflow6#1348
• rename to @timed / timed() to avoid collision with builtin timeit module/function
• use timeit internally instead of manually timing