trying to save memory in D2 calculation
trying to speed up D2 calculation by using less memory
remove trace
traces and timings in d2 calc
traces and timings in d2 calc
Made readgrid respect usespherical flag. If False, it reads cartesian arrays x_u etc and copies them over to the sperical arrays, lon_u etc.

mask is now assigned an array of ones if mask_rho isn't in the output file.
Merge pull request #7 from dcherian/devel2

Some changes to make tracpy better with idealized Cartesian models.
commented out xy test since does not actually work yet