Feature request: `str` <--> `ndindex` conversion

[bersbersbers]

ndindex looks super useful. What I love is the way to abstract from the actual index type, thus being able to use integer- and slice-based indexing using the same data type. I had previously struggled in my code to convert integers i to clice(i, i+1), but that was quite confusing when printing that representation back to the user...

Speaking of the user, I have an app where they enter indices in text, and I convert their input into a slice (or Slice/Integer, soon) and use it for some computation. It would be super useful to have a function that converts strings such as 5 or :5:3 into ndindex objects, and also to print ndindex objects back into such canonical string representation.

Given an array a and user input s as above, I would expect

eval(f"a[{s}]") == a[ndindex[s]]

if that makes sense.

Also, str(ndindex[s]) (or ndindex[s].foo()) should return pretty much its input, except for canonicalization.

[asmeurer]

There's some work in progress for this idea at #86. It needs some cleanup, but I think it should be doable and is definitely in scope.

I also agree that there should be a way to print indices in a way that uses direct Python syntax like 2:3 instead of Slice(2, 3).