Added doc strings, all, any, added some functionality to ndarray usinexisting stuff

Changed:

All cumulative operations are now prefixed with cum example cumsum, to distinguish between cumulative and on axis.
Moved most of the dunder math to _array_func implementations (the exception being pow). Added:
all and any functions to check currently require bool DType (we need to discuss applying truthiness to array members at some point)
The following methods were implemented for NDArray cumprod, cumsum, prod, sum, and mean(both with and without axis).

Mojo-Numerics-and-Algorithms-group / NuMojo