Fixes from cambridge

This PR contains three (hopefully) atomic commits that address three separate issues which came up during the bootcamp at Cambridge University on August 26th/27th. I'm happy to submit a new PR without one of them if people disagree with some subset of the changes.

#### 1. Commit *d90338a*

The last challenge in the loops lesson from `novice/shell` asks learners to figure out what the following loop does:

```bash
for how in frog11 prcb redig
do
     $how -limit 0.01 NENE01729B.txt
done
```

Many of the learners asked what the `-limit` flag does, which of course depends on which of our imaginary programs we are calling in this particular loop. I think that the presence of a flag here doesn't make much sense, so I changed the parameters to be three file names, which I think should be less confusing.

#### 2. Commit *3926b4d*

The last challenge in the conflicts lesson from `novice/git` asks learners what git does when there is a conflict in an image. Many of the learners tried this out themselves and were then unable to resolve the conflict. This commit adds instructions to the teaching guide for git on how to resolve these conflicts (or at least how I did it, someone with a higher level of git-fu might have a better suggestion)

#### 3. Commit *8e222dd*

The last challege in the functions lesson from `novice/python` asks learners to make a function that plots three graphs. Since at this point learners have only encountered functions that return something, many of them were having problems returning inappropriate things at the end of their functions. I just added a little note right before the challenge that functions don't have to return anything.