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This prevents issues like variables being passed as 'something; rm -rf *'
It also gives some robustness to spaces in file paths
Also good practice to use -n for non-empty strings and -z for empty strings

This also brings parallelism for none-GPU machines

Added '-list' flag to show tract names. Added option to select a subset of the pre-defined tracts from either Human or Macaque - user provides structure file with tract names or tract names and nsamples. Small bug fixes