closes #471 unclear question "How many species...?"

[Teebusch]

The species column has fewer unique entries than species_id. The relation between genera and species can be unclear to learners without biology background. Asking about genera fulfills the same learning objective but is less ambiguous.

[mondorescue]

@Teebusch FYI: I think your suggestions are sound and I would approve merging it, but to change the objectives, it requires the CAC to review it -- if I recall correctly.

@fmichonneau Am I correct?

[Teebusch]

@mondorescue I see. I'd argue that the learning objective doesn't change (inspecting the output of summarize to see how many levels a factor has). It's simply asking for a different factor (genus instead of species), because there's some ambiguity regarding species and species_id.
This has tripped up multiple instructors (including me, as I went through the lesson), so I'd hope it can be resolved quickly.

[Teebusch]

I think this PR can be rejected/closed, since the challenge has changed with #663
The challenge has moved to a later point in the lesson, after the concept of factors has been introduced, and is now asking for the number of genera. The challenge is now:

1 Change the columns taxa and genus in the surveys data frame into a factor.
2 Using the functions you learned before, can you find out...

• How many rabbits were observed?
• How many different genera are in the genus column?

Learners are expected to use the summary and nlevels functions to answer the questions

surveys$genus <- factor(surveys$genus)
summary(surveys)
nlevels(surveys$genus)

As I said in my previous comment, I don't think this changes the learning objective, just the implementation. So I don't think it needs to be approved by CAC. But maybe you guys disagree?

(note: there's a little markdown formatting issue in the challenge that needs to be fixed)

[fmichonneau]

thanks!