mrview: fix handling of -focus and -target options
mrview: fix handling of -focus and -target options

- The -focus option was not labelled as type_sequence_float(), leading to
  a warning if the first value is negative, e.g.:

      mrview: [WARNING] Value "-1,2,3" is being used as the expected argument
      for option "-focus"; is this what you intended?

- It should be allowed to use the -target option multiple times.

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