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.
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.

dcherian/tracpy,kthyng/tracpy