Prefer functions over methods.
Most methods on NumPy arrays also have an equivalent NumPy functions that takes the array as input. I have noticed that many novice programmers prefer function (which have a pretty clear input-output relationship) to methods (which have an implicit input). This commit changes all instances of calling methods on arrays to calling the equivalent NumPy function instead. I tried to simplify the explanation in `01-numpy.md` as well, which hopefully does not significantly affect the flow of the prose. I also added a "function" entry to the glossary and linked to it in `01-numpy.md`.