rework of temporal sampling

[Datseris]

I think we can rework the source code that handles temporal sampling, to return units that are multiples of existing Julia types. E.g. Hour(1) for hourly sampling, Day(1) for daily, Hour(3) for 3-hourly sampling, etc. This will not only bring more type stability, but also reduce amount of source code as well allow naturally handling arbitrary multiples of such samplings, such as weekly Day(7), or centurial Year(100).