Data retrieval doesn't return the same dates for test

[pwdemars]

In data_retrieval.py the SEED variable is defined but not used to set a random seed. The subset of days held out for testing therefore differs each time the script is run.

Probably easier to just define the test days manually to be honest. Those used in publications are:

['2016-01-12',
 '2016-07-25',
 '2016-11-21',
 '2017-03-18',
 '2017-04-07',
 '2017-05-12',
 '2017-05-26',
 '2017-06-25',
 '2017-12-18',
 '2017-12-30',
 '2018-01-15',
 '2018-03-08',
 '2018-03-18',
 '2018-05-30',
 '2018-09-11',
 '2018-11-13',
 '2019-04-03',
 '2019-05-22',
 '2019-10-30',
 '2019-11-09']