Parse date fields to be in correct format

[wibo-pipping]

START_DATE and END_DATE fields cause an error when loading with transmart-copy.

Insert into i2b2demodata.observation_fact   4% │███████▌                                                                                                                                                                                     │   50/1241 (0:00:01 / 0:00:25)
2018-03-15 11:16:00,278 [ERROR] Error processing row 52 of i2b2demodata/observation_fact.tsv: Text '2005-11-15' could not be parsed at index 10
2018-03-15 11:16:00,293 [ERROR] Text '2005-11-15' could not be parsed at index 10

Add validation to START_DATE and END_DATE fields

[wibo-pipping]

@jochemb Could you unassign this issue from my name?