in simplegrid.m: implemented 'centered' property; fixed bug in slanted, which had different values for longitudes 0 and 360; revised tests, added center_resample and sh; in constructor, added sanity check for different values for longitudes 0 and 360; fixed interpolant; added sanity to stop interpolation of centered grids
in simplegrid.m: implemented 'centered' property; fixed bug in slanted, which had different values for longitudes 0 and 360; revised tests, added center_resample and sh; in constructor, added sanity check for different values for longitudes 0 and 360; fixed interpolant; added sanity to stop interpolation of centered grids

jgte/orb,jgte/orb,jgte/orb,jgte/orb,jgte/orb