Exploratory code for a time series library

Concepts:

• UntypedTimeSeries as a minimal interface for a time series
• SteppedUntypedTimeSeries as a discrete time series which can be inspected for its start, end and interval
• Specific interfaces for typed variants of kilowatts, kWh, unitless, etc. (since Java does not have template specialization)
• iterators to get balls-to-the-walls performance