Corrected interpolate.m and ad_tinerpolate.m

[arango]

Corrected a bug in interpolate.m and ad_interpolate.m, which are used to compute the input functionals for 4D-Var observation impacts and sensitivities for the desired metric.

The problem is in the call to the function sw_dist from the seawater package. We need

     dis = cumsum([0; sw_dist(Ypath(:),Xpath(:),'km')]);

instead of

     dis = cumsum([0; sw_dist(Xpath(:),Ypath(:),'km')]);

since the ordetr of argument is sw_dist(lat,lon,...).

Many thanks to Andy Moore for bringing this to my attention.