imaging: New primitive to apply base position offsets.

This is for moving targets so that WCSMOSAIC will create a mosaic
where the moving taget is a point source.  I was a little surprised that
I could not find an existing primitive for this.
WFCAM: New recipe for fast-moving asteroids

This is variant of the JITTER recipe, whereby the relative base
positions are used to apply offsets to the WCS, to enable the mosaic
to show the asteroid as a point source, rather than trailed.  A
consequence of this is that the accurate astrometry cannot be derived
and the mosaic has an additional SKYBASE WCS Frame.
WFCAM: Add SHIFTBASE argument to invoke _SHIFT_TO_BASE_POSITIONS.

Also disable auto astrometry when SHIFTBASE is true.  It doesn't work
for trailed stars.