Create cases json fails if growth metric days is set to 3

[mattparrilla]

Works when default is used:

(venv) matt@matt ~/git/chartingcovid [new-cases]$ python create_covid_json.py --growth_metric_days 3
Traceback (most recent call last):
  File "create_covid_json.py", line 423, in <module>
    args.growth_metric_days, args.output_fips_first)
  File "create_covid_json.py", line 397, in generate_json
    county_csv, state_data, growth_metric_days, output_fips_first)
  File "create_covid_json.py", line 383, in generate_covid_data
    growth_metric_days, latest_date, total_days_of_data)
  File "create_covid_json.py", line 227, in record_growth_metrics
    doubling_time = get_averaged_doubling_time(preceding_case_counts)
  File "create_covid_json.py", line 106, in get_averaged_doubling_time
    return mean([get_doubling_time(r) for r in growth_rates])
  File "/usr/lib/python3.6/statistics.py", line 311, in mean
    raise StatisticsError('mean requires at least one data point')
statistics.StatisticsError: mean requires at least one data point