Add conversion routine for annual data.

The routine converts annual data into quarterly data.
Currently two options are implemented:
- equal-per-quarter setting values in each corresponding quarter to
corresponding annual value
- cubic-spline applicable ONLY to flow variables

(cherry picked from commit 53ec34790610f1d8a400b1376876cf1bda3f8395)
(cherry picked from commit cdf249e78121dea324998f0c0e3211221c78682f)