Add an implementation of nutation.

Note that we use the exact method always, unlike in the
old implementation, where we use an approximation. However,
even the exact method here should be much faster than the
trig in the old method.

Note also that this implementation doesn't have a test yet, since
it's getting its numbers from a function which is known to give
incorrect results. When the AstroVars tests pass, then we
should add a nutation test.

CCMAIL: akarshsimha@gmail.com
The problem was caused by not converting to radians before doing trig.
Add a nutation test
Remove warning about incorrect code which has been corrected
Change from using quaternions to matrices.

This is rather unfortunate, but unfortunately we need to
be using orthogonal matrices instead of quaternions.
The reason is that the conversion from equatorial to horizontal
coordinates has a sign change, with the result that the determinant
is -1 instead of +1. This means that that map cannot be represented in
the usual way by quaternions, so we need to use matrices instead.

CCMAIL: akarshsimha@gmail.com
Add an implementation of Eq-Hor conversion.