Missing impl of general global ops and corresponding nan cases

[ehsanmok]

@bluss I'd like to propose implementing min/max and nan_min/nan_max and alike (sum, mean, median) for ndarray to be more compatible with numpy. (I'll create a list of task if confirmed later!)

I have the impl of nan_min/nan_max ready returning Option<A> where None is for empty array, Some(number) if not all are NAN and Some(NAN) if all entries are NAN (f32 or f64). Is that ok or should panic at the last case?

Impl of generic min/max seems non-trivial and needs Any::TypeId checking for float and non-float?

I'd also like to propose changing scalar_sum to sum.

P.S. related to this.