Started adding support for caches when reading data. Cache files for

Started adding support for caches when reading data. Cache files for
daily data calculations works already. (Cache done during first
calculation when the cache does not yet exist.)

Cache does not yet handle situations if the daily data is updated once
the cached value has been calculated.

Memory leak fixes and api changes.

Signed-off-by: Mika Laitio <lamikr@pilppa.org>