Slicer _cmd revision and quotes around %s

[andrewcyung]

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Summary

Fixes #2918

List of changes proposed in this PR (pull-request)

• changed the command line string used to call the BRAINSresample utility that is included in 3D Slicer. The master branch version will not properly call the BRAINSresample utility if the user's PATH does not specifically include the full path to the command line utility. Using "Slicer --launch <name_of_Slicer_command_line_utility>" is the correct way to call it.

• add quotation marks (with escape sequence ") to the format specifier argument when parsing file path inputs, in order to allow filenames with spaces in them.