added unified coordinate conversion routines and planetary positions

This commit revises the routines in isisscripts for the coordinate
conversions. All routines are contained in src/slang/astronomy/coco.sl.
They allow to be called with vector and angular arguments and all
use matrix based routines internally.

The commit also includes a first set of routines to calculate
topocentric positions of the planets, following the algorithm used by
the Astronomical Almanac and based on VSOP87.

In total, the following new functions are made available:

Coordinate conversions:
equatorial2horizon, horizon2equatorial
equatorial2galactic, galactic2equatorial
equatorial2ecliptical, ecliptical2equatorial

Precession of equatorial coordinates:
precess

Planetary positions:
planetpos