[Add Check]: check session start time

[CodyCBakerPhD]

Should probably check the session start time has been set properly within a file; not just as a datetime object, but as a valid date range for performing neurophysiology studies. E.g., if session_start_time <= datetime(1970, 1, 1) print "Session start time may not be set to the true date of the recording."

[oruebel]

You probably also want to have a similar check for timestamp_reference_time, with the difference that timestamp_reference_time == datetime(1970, 1, 1) should not raise an error since some labs are using Unix time as reference to have consistent timestamps across file.