added examples to the first two episodes of python ecology class from…

[ezgirmak]

Hi,

As part of my instructor checkout procedure, I decided to embed examples back into the lesson materials from the instructor notes as discussed in the issue #361. I did the first two episodes and tried to follow carpentries style as much as possible. If everything looks good, I can continue to do the rest of the episodes.

Here are some issues related to the second episode.

• Second challenge in grouping challenges has a further example in the instructor notes 'I want to know the last survey year, median foot-length and mean weight for each plot/sex combination' Should I add it as an additional challenge?

• The instructor notes and the example in the lesson for Calculating Statistics Challenges is not the same. In the instructor notes, the name of the variable is plot_names, in the example it is site_names. The code to get number of unique species in the data in the instructor notes is len(pd.unique(surveys_df["species"])) and there is no column named species. So I wrote it as
  len(pd.unique(surveys_df["species_id"])) in the lesson solutions. I have not changed the notes but I can change them in my next commit.

Thanks,
Ezgi

[maxim-belkin]

Thank you, Ezgi! To speed up the review/discussion process, could you please split your pull request into smaller ones (solution-per-PR)? I linked a quick guide on how to do that but please let me know if you experience any difficulties along the way.