Add cyclic forcing support

[SouthEndMusic]

Fixes #153.

Notes:

• With timeseries which are extrapolated in a periodic manner, the first and last data values must be validated to be the same. For linear interpolations this ensures continuity, and for constant interpolation this value would be otherwise ignored.
• Time periods like years and months do not always have the same length, so this can get out of sync when these time periods are repeated. This should at least be documented well, and might need a follow-up issue.