regulate meta-recordings duty cycle data upload

[kdarras]

At the moment, upload of metadata fails if the two duty cycle columns are not filled with positive integers. However, meta-recordings do not need to have duty cycles (most don't), so the entry of duty cycle info should be optional.

However, if duty cycle data is provided, it must be provided for both fields (duty_cycle_recording and duty_cycle_period).

Furthermore, input of date-times in the future should not be allowed, and duty_cycle_recording cannot be larger than duty_cycle_period.