gbin: new package GBIN for Gaia internal data

The GBIN package attempts to read GBIN files, which are the
java-serialised-object based data files used internally within
DPAC for Gaia data storage.

For avoidance of confusion: this is Gaia the astrometry satellite,
not Gaia the image analysis tool.
gbin: add TableBuilder implementation
gbin: make STIL and applications aware of GBIN
gbin: don't call getCount!

The GbinReader.getCount method looks tempting, but it works by
reading the input file all the way through, so it is (a) slow and
(b) repositions the InputStream near the end.  Steer clear!
gbin: improve table reading and add configurability

The table reader can now read some of the metadata, including row count,
from the GBIN file (GbinReader.getGbinMetaData).
It is smarter about naming columns; names don't have to have the entire
object hierarchy if they are unique anyway (though you can configure it
to foce fully hieararchical names).
And field names are (optionally) sorted alphabetically,
rather than getting them in the random order that reflection gives you.

The various options are controlled by a "profile" which tells the
GbinTableBuilder how to turn a GbinTableReader into a StarTable.
A default implementation is supplied.
gbin: add javadocs

Comment and tidy up the code.  It's now basically ready for use,
though not yet documented in client packages.
gbin: add user-level documentation

The GBIN format is now documented in the STILTS and TOPCAT user
documents.

The documentation text is stored in the GBIN package
and imported into SUN/253; the same could be done for SUN/256,
but at present the other formats are just documented in a couple
of lines there, so follow the same practice.  That was done
(presumably?) to avoid repeated text, but importing the text
from an external package like I have done here would make it
work.  Maybe fix it for the other formats one day.