Python 3 compatibility for functor division

This follows up on !2214 (merged) which introduced the __truediv__ decoration of functors but missed __rtruediv__.

In addition, fix math (stdlib) import in LoKiCore.math (functors that call math.log will fail)

Must be applied together with Moore!592 (merged) (math.log -> log).

See LHCBPS-1858