Moves memory allocation into constructors of (almost) all couplers. See #140.
Now we do the right thing when interpolating periodic data. See #139.

On the downside, *all* periodic data is read in at once now. (I.e. one
should not expect periodic data with thousands and thousands of
records to work smoothly.) This should not cause any issues, though.
More fixes (proper implementations of precip_time_series needed by the new PDD code).
More fixes (snow_depth==0 in the ocean; fixes memory allocation in PO_delta_SL).
Closes #121.
Fixes units and metadata in PA_delta_P.