Axes: `axis="columns"` versus `axis=1`

[trevorcampbell]

An alternative here is to say axis='columns' and then emphasize above that when we are applying a function row-wise it will go across all the columns which links to the name of this option (so that students don't think it should be called "rows" since it is "row-wise"). I'll let you pick what you think is clearer, just a possible alternative, I generally like naming the axes with a string instead of a number personally since I forget which is axis 0 and 1 constantly... (and I think there is some pandas function where they are confusingly inverted). There is a couple of occurences of this further down in case you decide to change it

Originally posted by @joelostblom in #301 (comment)

[trevorcampbell]

Need to check how this would propagate throughout the book before doing. Not necessary for 1st edition.