add pandas df filtering exercise

mentioned in merge request !12 (merged)

Nice exercise. Like it. I could not test the compilation.

Remark on solution:

Proposed solution leads to UserWarning, as index array of second "filter" is too large for result of first filter.

In [9]: covid_cases[covid_cases['year'] == 2019][covid_cases['cases'] > 0]                           
<ipython-input-9-de53dbbe1f91>:1: UserWarning: Boolean Series key will be reindexed to match DataFrame index.
  covid_cases[covid_cases['year'] == 2019][covid_cases['cases'] > 0]
Out[9]: 
         dateRep  day  month  year  ...  geoId  countryterritoryCode   popData2019 continentExp
5181  31/12/2019   31     12  2019  ...     CN                   CHN  1.433784e+09         Asia

[1 rows x 11 columns]

Proposal: Combine index arrays with booleans

c = covid_cases                                                                              
c[(c['year']==2019) & (c['cases']>0)]

Explanation:

In [1]: import pandas as pd                                                                          

In [2]: covid_cases = pd.read_csv('https://git.embl.de/grp-bio-it-workshops/intermediate-python/-/raw
   ...: /master/data/CovidCaseData_20200624.csv')                                                    

In [3]: c0 = covid_cases                                                                             

In [4]: c1 = c0[c0['year']==2019]                                                                    

In [5]: c2 = c1[c1['cases']>1]                                                                       

In [6]: c1[c0['cases']>1]                                                                            
<ipython-input-6-81a87b0cc528>:1: UserWarning: Boolean Series key will be reindexed to match DataFrame index.
  c1[c0['cases']>1]
Out[6]: 
         dateRep  day  month  year  ...  geoId  countryterritoryCode   popData2019 continentExp
5181  31/12/2019   31     12  2019  ...     CN                   CHN  1.433784e+09         Asia

[1 rows x 11 columns]