Updates for ndarray 0.13

[danwilhelm]

The ndarray crate was updated to 0.13.

This issue and corresponding PR:

1. Replace Array::from_vec() with Array::from(), since it is now deprecated.
2. Replace manually calculated approximations with approx traits, the new recommended method for comparisons.
3. Add the approx feature flag and approx crate to Crates.toml.
4. Move simpler examples first. Update explanations for clarity.
5. Rename vector-sum.md to vector-comparison.md, since it now mostly demonstrates the approx crate.