mrview Connectome tool: Fix streamtube initialisation
mrview Connectome tool: Fix streamtube initialisation

Failure to initialise member bool Connectome::have_streamtubes could lead to memory issues, as reported in #1330.

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3