2.3.1 Change of internal error handling.

Before a error handler was installed by init_pygsl into gsl which translated the error code (and the message) to a python exception. This required that the GIL was available, which numpy ufuncs dispose. Thus now this gsl error handler is deactivated and instead the C error code returned by the C function is translated to an error code by the wrapper called from python.

UFuncs do not call this handler now at all.