doc improvements
doc tweaks
start on packages apparent and solar
make Horner variadic
replace private c2000 with base.J2000Century
add Position function
rename functions,
split an example,
simplify a conversion from seconds to radians.
new const SmallAngle
Big cleanup and reorganization.  New types Precessor and EclipticPrecessor
allow separation of the steps of computing parameters from epochs and
computing precessions from coordinates.  More precise types for proper
motion clarifies units.  Documentation errors fixed.
complete package apparent