Expand time series support

[mnlevy1981]

Right now, we only support time series files containing exactly one year. (Looking at the logic in the code, it's possible that a 2-year time series file would also be treated correctly but that hasn't been tested.) Other projects often make 5-year or 10-year time series files, so it would be far better to support an arbitrary number of years in each file.

This will require some invasive changes to the code, so I'm not going to address it right away... but I think it's a pretty important step for getting other projects to use this package.