document out_vars string options for IO thorns provided by IOUtil

[Roland Haas]

The IO thorns typically support setting some options on a per variable basis using option string embedded into their out_vars parameters, eg (from https://bitbucket.org/einsteintoolkit/einsteinexamples/raw/master/par/GW150914/GW150914.rpar):

IOHDF5::out_vars                        = "
  Grid::Coordinates{out_every=1000000000 refinement_levels={0}}

which override the values given in parameters.

These are all handled by IOUtil in its SetOutputVar routine https://bitbucket.org/cactuscode/cactusbase/src/80544e08d406424e8bd1e21e5574218bd7239843/IOUtil/src/Utils.c#lines-742 which contains one Util_TableQueryValueInfo per option.

IOUtil should document these options (or all the IO thorns should and they should at least document which options they support, eg only the Scalar and Basic like IO thorns will support the reductions option.).

[Roland Haas]

hackathon (documentation writing)