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