Add "--participant_label" command line argument

[chrisgorgo]

To allow BIDS Apps to parallelize over participants (remember that the input directory is the root of the dataset - not a single subject) each app need to support --participant_label command line argument. This optional argument has the following properties:

• When not included analysis should be done on all participants
• When included it should take a label of the participant you want to analyze. That label should not include the sub- prefix that is present in the folder names. For example --participant_label 01
• The argument can also take a space delimited list of labels. For example --participant_label 01 03 10.

More about command line arguments and parallelization can be found here.

[Chaogan-Yan]

Solved.