Support pipelines descriptions in the Boutiques format.

[glatard]

Could be used:

• To describe pipelines more formally (relations between inputs, input validation schema).
• To import/export pipelines. BIDS apps can be imported in Boutiques semi-automatically.

See more info at https://github.com/boutiques/boutiques

[glatard]

Issues:

• [ ] Boutiques pipelines don't have canExecute boolean (platforms could use a custom property).
• [ ] Boutiques pipelines don't have error codes and messages (but they could). Related to https://github.com/boutiques/boutiques/issues/36. Fixed in https://github.com/boutiques/boutiques/pull/196#issuecomment-357397768
• [x] Boutiques pipelines now have properties (https://github.com/boutiques/boutiques/pull/194).
• [x] Existing CARMIN pipelines may not have a container image (but it's not required in Boutiques).
• [ ] Existing CARMIN pipelines may not have a command-line template.

Once the limitations above are solved, it should be possible to automatically import a CARMIN Pipeline object as a Boutiques descriptor.

[cmaumet]

This was discussed on the CARMIN call on January 10th, 2017, cf. minutes.