Use sane defaults for paths

[kdpenner]

A few issues.

1. I installed the package in a conda environment. For every line in patients.py with

os.path.join(os.path.dirname(__file__), "../data/"+string)

the path has the prefix /Users/kpenner/miniconda3/envs/rtlive/lib/python3.9/site-packages/covid/../data/

Some place like ~/.local/share/covid/data/ is logical.

2. The tutorial should mention that you need to use download_patient_data() before you do anything.

3. I have to untar latestdata.tar.gz. If I don't and pass the .tar.gz here:
   

   covid-model/covid/patients.py

   Lines 27 to 32 in cf892d5

   	patients = pd.read_csv(
   	file_path,
   	parse_dates=False,
   	usecols=["country", "date_onset_symptoms", "date_confirmation"],
   	low_memory=False,
   	)

   
   pandas throws ValueError: Passed header names mismatches usecols. If I drop usecols, the output of patients.columns is Index(['././@PaxHeader'], dtype='object').