Fixes from cambridge

[rbeagrie]

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:

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.

[wking]

On Thu, Sep 11, 2014 at 10:39:18AM -0700, Rob Beagrie wrote:

1. Commit d90338a

The last challenge in the loops lesson from novice/shell asks learners to figure out what the following loop does: …

I haven't looked over the content of these commits in detail, but ideally useful discussion like this should live in the commit message body. That way folks have the explaination/motivation/notes available whenever they look at the commit, instead of having to track their way back to this PR and read the notes here. For advice on laying out a good commit message, see Tim Pope's model commit message 1.

[rgaiacs]

+1 to merge after small corrections.

@rbeagrie Thanks very much for the note about git checkout --ours/theirs. I didn't know about it. =)

[rbeagrie]

@wking duly noted - next time I'll put some more detailed justification in the commit message!

[gvwilson]

@karinlag Can you please have a look?