Plotting a line of fit

Reword Plotting the line of best fit exercises to avoid using the phrase best fit. Also add two more exercise types in which there is a strong non-linear trend in the data so a line in not appropriate.

For Trello card: https://trello.com/c/7BAn7nbX/278-line-of-best-fit

I still have to add further exercises asking students to assess a line of fit.