Separate measurement_values into data_reduction records in database

[squaregoldfish]

With the new PERIODIC measurement mode for the SensorValuesList, it is theoretically possible that an Instrument has two different variables for the same Measurement (i.e. at the same time) that would use values from the same SensorType but at different temporal resolution. One example could be that one Variable is measured periodically and another continuously. So the periodic variable will need to average groups of values while the continuous one would use the instantaneous measurements.

At the moment the first Variable found in the DataReductionJob determines how the values for a given SensorType are extract for that measurement, whereas in reality the different setups should be recorded in the data_reduction records.

This in turn will drive a discussion of what exactly a Measurement represents, both in the database and in the code.