Teach how to create a new column in Ibis?

[joelostblom]

Currently, in the textbook we show how to filter data using Ibis, but we don't show how to create new columns. Filtering the data looks the same as in pandas, but creating a new columns uses mutate instead of assign. In the reading tutorial, we both create a new column and filter the data. This means that it now matters which line calls execute to create the pandas dataframe from the database, since students have to do that before getting access to assign. We are solving this now by calling execute up front, but this is teaching students a bad habit since it creates an unnecessarily big data frame and negates many of the benefits with working with a database and ibis. Also see https://github.ubc.ca/UBC-DSCI/dsci-100-instructor/issues/836

[trevorcampbell]

I closed this as part of #302 -- I thought we didn't cover mutate/assignment in Ch1, so wasn't easy to discuss mutation/assignment for databases in Ch2. But I'm re-opening this because I discovered that we do indeed teach mutate in ch1 now, so it's possible to refer to that in Ch2 in the context of DBs.

(But I am leaving this out of 1st edition -- it's a minor change and not super important to get into print)