Better map sampling

[kadrlica]

@erykoff suggests an improvement to the map sampling scheme.

        nside = healpy.get_nside(hpxmap.data)
        ipring,=np.where(hpxmap.data > healpy.UNSEEN)
        thetaMap,phiMap = healpy.pix2ang(nside, ipring)
        lonMap = np.degrees(phiMap)
        latMap = 90.0-np.degrees(thetaMap)
        hi,=np.where(lonMap > 180.0)  # FIXME
        lonMap[hi] -= 360.0
        lon = np.linspace(lonMap.min(), lonMap.max(), xsize)
        lat = np.linspace(latMap.min(), latMap.max(), xsize)
        lon, lat = np.meshgrid(lon, lat)