doxygen -xml should be created by python bindings build system

[richard]

Currently, the doxygen rules for the API documentation in the main build are called with "--xml" to generate xml output as a side effect, and this is later used by the python bindings to generate the documentation comments file.

Instead, it would be much cleaner for the python bindings build system to call doxygen itself, to generate the xml output when it's needed.

[olly]

Or perhaps we just wait for the swig doxygen stuff to get merged and use that?

[richard]

That sounds like a reasonable approach, if it's going to be merged in the reasonably near future (which it sounds like it might).