Update method used to run line-profiler

[LouiseABowler]

Fairly recently, the line-profiler project implemented a new way of running the profiler, which is now recommended in their quick-start guide. The method currently used in the lesson, which calls kernprof explicitly, will be removed from line-profiler's guidance at some point in the future, so we should make the switch before then.

See here for the current and legacy versions in line-profiler's quick-start guide.

[Robadob]

Will have to check that env var approach is suitable across windows/linux but seems like a reasonable update (I was never a huge fan of calling a different named package). Although it outputs to file, so you lose the rich text.

[LouiseABowler]

I made a start on this today, will test out on Linux and Windows next week before going any further with the PR. I'll check if there is a way to output the rich text version immediately too - you do get an lprof file that has the nice formatting once opened, but I agree, it's a nice output to see right away if possible.