Modified Tractography::WriterBase to check presence of output file
Modified Tractography::WriterBase to check presence of output file

When creating a new track file, the WriterBase class will now abort if the file already exists, unless the -force option has been provided; this makes behaviour similar to image creation.

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