Add Send + Sync to NdFloat
Add Send + Sync to NdFloat

This is a non-breaking change because these traits have no methods (no
naming conflicts can occur), and the only legal NdFloat impls are f32
and f64, so no impls can break.

Pireax/rust-ndarray,SuperFluffy/rust-ndarray,SuperFluffy/rust-ndarray,Pireax/rust-ndarray