Use `ST_distance_sphere` as default instead of `ST_distance_spheroid` for geodetic distance calculations.

[jbronn]

As brought up by kmishler in this ​django-users thread about GeoDjango geodetic distances, ST_distance_sphere performs faster than ST_distance_spheroid. My informal tests suggest that using ST_distance_sphere yields results ~30-40% faster that are a maximum of 0.25% less accurate.

The attached patch changes the default to ST_distance_sphere -- spheroid calculations may be enabled by setting the distance_spheroid keyword to True in the geographic model definition (e.g., point = models.PointField(distance_spheroid=True)). I haven't committed this yet because I wanted to solicit comments on whether there's a better way to do this.

[jbronn]

Patch for using ST_distance_sphere by default.

[jbronn]

Fixed in r7641. ST_distance_sphere is used by default. However, the mechanism to use ST_distance_spheroid is more dynamic than the original solution in the patch attached to this ticket.

For the distance lookup types the following syntax will enable spheroid calculations:

qs = City.objects.filter(point__distance_lte=(pnt, D(km=5), 'spheroid'))

When using GeoQuerySet distance method, the spheroid keyword just needs to be set:

qs = City.objects.distance(pnt, spheroid=True)