Check class of datetime in sample data

[samzipper]

The class of the datetime column varies among sample datasets:

• in calibrated_stic_data, it is character
• in classified_df, it is posixct
• in tidy_stic_data it is posixct

I think it should be switched to posix in calibrated_stic_data unless there is a reason that it is not?

[christopherwheeler]

changed calibrated_stic_data datetime col to posixct