Changes to episode timings and instructor notes

I've had a go at adjusting the timings for each episode, largely to

Match sessions to the 2 hour 50 minute threshold for each session (session 1: episodes 1 and 2, session 2: episode 3, session 3: episodes 4 and 5, session 4: episodes 6 and 7). I believe these are the session/episode splits used in all previous rounds of teaching.
Increase the proportion of the time spent on challenges, particularly for the first 3 episodes as per the feedback. I went through the challenges to check and have also increased the amount of challenge time for episode 4 - the challenges in this episode are quite involved. Otherwise, the timings for each episode and teaching/challenge splits have been increased roughly proportionally to the original timings.
Have also filled out the instructor notes and included these timings with a note about breaks. This may need to be checked in detail because I haven't taught the course so may be missing things!

To double check, will changing the yaml parameters at the start of each episode like this change the home page timings?

Addresses #112, #133, #145

carpentries-incubator / high-dimensional-stats-r