The reason for this is, that we can do the file IO very transparently by forwarding the calls to the native HDF5 library. In principle, it is wished to have the possibility of downsampling the data before writing them to disk or sending them over the net. Any other visualization using the DS has 'just' to generate requests and collect the data. A HDF5-style API should make this easy. An easy API should make this simple to implement.
Possible parameters depend on thorn to be invoked.
Possible parameters depend on field type and visualization needs. If request cannot be handled by anyone, an error has to be generated. The thorns themself have to parse the contents then, and do something according to its contents. If something goes wrong during parsing etc., the thorn now has to generate an approriate error message.