Support for STC within AST

[David Berry]

I have modified the Starlink AST library to read XML instances of the
draft V1.20 STC specification and create corresponding in-memory AST
objects, which can then be manipulated using all the facilities of AST.
There are demonstrations of some of facilities this provides, together
with documentation, download, etc, on the AST homepage
(www.starlink.ac.uk/ast). These demonstratons allow you to:

- Test points to see if they are inside or outside the region described by
a given STC instance.

- Test two STC instances to see if they overlap (they need not refer to
the same coordinate systems or units - for instance you can test for
overlap between a ICRS/TT/FREQ region and a L/B/UTC/WAVELEN region.

- Plot the spatial outline of an STC region over a grid defined by a set
of FITS-WCS headers.

- Convert coordinate values between the coordinate systems and/or units
represented by two STC instances.


A number of parts of the current STC specification are not currently
supported. These include (amongst others):

- No support for 3D space coordinate systems. <SpaceFrame> elements can
only be read if they describe a 2D celestial longitude and latitude axes
using one of the following space reference frames: ICRS, GALACTIC_II,
SUPER_GALACTIC, HEE, FK4, FK5, ECLIPTIC.

- Velocities associated with SpaceFrames cannot be read.

- Any <GenericCoordFrame> elements within an <AstroCoordSystem> element
are currently ignored.

- Any <OffsetCenter> found within a <SpaceFrame> is ignored.

- <SpaceFrame> elements can only be read if the reference position is
TOPOCENTER. Also, any planetary ephemeris is ignored.

- Regions: there is currently no support for STC regions of type Sector,
ConvexHull or SkyIndex.

- <RegionFile> elements are not supported.

- No support for referring to external files (FITS or XML).

(etc)

Please report any bugs, problems, comments, etc.

David Berry