#16 Add "data object" subtype to the tools XML

[alexbruy]

Thanks to Volker, now "data object" subtype outputed in the plain text tool description
(see https://sourceforge.net/p/saga-gis/code/ci/bbbb7ee62a7eca9d904d512b111cc3169c90ec5a/). Here is a patch to add this to XML description.

[Johan Van de Wauw]

Thanks for your pathc - applied to master.