Harvest support range dates separated by dashes

[dharizza]

Harvest sources https://data.cdc.gov/data.json with temporal coverage dates that use dashes as separator are failing with the following error:

Error: Call to a member function getTimestamp() on null in       [error]
DatajsonHarvestMigration->prepareRow() (line 201 of
dkan/modules/dkan/dkan_harvest/modules/dkan_harvest_datajson/dkan_harvest_datajson.migrate.inc).
Error: Call to a member function getTimestamp() on null in DatajsonHarvestMigration->prepareRow() (line 201 of dkan/modules/dkan/dkan_harvest/modules/dkan_harvest_datajson/dkan_harvest_datajson.migrate.inc).
Drush command terminated abnormally due to an unrecoverable error.       [error]
Error: DateTime::__construct(): Failed to parse time string
(1899-2017) at position 6 (0): Unexpected character in
dkan/modules/dkan/dkan_harvest/modules/dkan_harvest_datajson/dkan_harvest_datajson.migrate.inc,
line 189

This is because dkan_harvest doesn't support temporal coverage values following the format "YYYY-YYYY". As it is a common format, in this PR we are adding support for it, so any harvest source with a temporal coverage value using the format YYYY-YYYY should work correctly just as any other YYYY/YYYY rage dates.

How to reproduce

1. Run harvest for a harvest source with temporal coverage values using the format YYYY-YYYY, it should fail with the message above. For example in https://data.cdc.gov/data.json the dataset with identifier https://data.cdc.gov/api/views/66i6-hisz

QA Steps

• Run harvest for a harvest source with temporal coverage values using the format YYYY-YYYY, it should work correctly, the message from above shouldn't stop the execution of the harvest. For example in https://data.cdc.gov/data.json the dataset with identifier https://data.cdc.gov/api/views/66i6-hisz