add pandas df filtering exercise

for context: the dataframe mentioned in the exercise is built from this CSV, i.e.

covid_cases = pd.read_csv('https://git.embl.de/grp-bio-it-workshops/intermediate-python/-/raw/master/data/CovidCaseData_20200624.csv')

one possible solution would be:

covid_cases[covid_cases['year'] == 2019][covid_cases['cases'] > 0]

Happy for review from anyone @ralves @meechan @ext.bauer

_episodes/02-data.md

@@ -39,6 +39,12 @@ keypoints:
 > Use what you've learned to find all rows in the `covid_cases`
 > dataframe that report cases in the year 2019.
 >
+> > ## Solution
+> > ~~~
+> > mask = (covid_cases['cases'] > 1) & (covid_cases['year'] == 2019)
+> > print(covid_cases[mask])
+> > ~~~
+> > {: .language-python }
 {: .challenge }
 
 {% include links.md %}