bedpostx_gpu cannot be run locally

Reported at https://www.jiscmail.ac.uk/cgi-bin/wa-jisc.exe?A2=FSL;29041c9a.2309

At the moment bedpostx_gpu can only be run on a system where fsl_sub has been configured to have a cuda queue.

This can be worked around by specifying a dummy queue, e.g.:

bedpostx_gpu -Q dummy ...

But this limitation really should be fixed. @sjbabdi @ndcn0236 any thoughts?