ArrayRef instead of ArrayBase in function interfaces #563

Hi,

I frequently find myself wanting a read-only array in Python backed by a Rust-owned buffer (usually to print it in an interactive session).
For this I use PyArray::borrow_from_array which takes a &ArrayBase as parameter.
I would like this function to take a &ArrayRef instead to be able to use it with a greater variety of arguments.

While this is a breaking change, I reckon the necessary code changes at call sites would be small, since ArrayBase already implements Deref<Target=ArrayRef>.

In my quick search I only found a couple of places, where this proposed change could be applied

• PyArray::borrow_from_array,
• PyArray::from_array,
• impl ToPyArray for ArrayRef (? not sure about this one).

Another consequence would be to bump the minimum version of the ndarray dependency to 0.17.1.

Kind regards!