+ Separate the test for significant gravitational lensing around the
sun from the actual lensing calculation. Now there are two methods
-- checkBendLight() and bendlight(), the former doing the job of
checking whether the skypoint is close enough to the sun to be
lensed by its gravity.

+ [Major] Recompute coordinates of a SkyPoint only when about a minute
has passed since the last recomputation. This means that precession,
nutation, aberration, are calculated only when there are
sufficiently large changes in time. This is expected to increase the
speed of recomputing the skymap, although I have not tested the
improvement.

This is also a potentially dangerous change. I've not tested to
check that precession is recomputed when the step is sufficiently
large.