Basic support for XNAT Pipelines

[jhuguetn]

This PR includes basic functionalities for running and monitoring XNAT pipelines.
Some CI tests for the added methods are also included to keep the coverage ratio.

[jhuguetn]

PR branch rebased

[jhuguetn]

PR branch rebased (again)

[jhuguetn]

PR rebased and ready to merge @xgrg

[xgrg]

Thank you @jhuguetn. We already have discussed most of it offline.
One last question though: can you briefly comment on why the add function was removed?

[jhuguetn]

Few reasons,

• Function is currently not working nor under Unit Testing control.
• Functionality is not listed as a supported one in the XNAT API directory (see Project Pipeline Management API).
• Generally speaking I don't like adding administrative features to pyxnat. At this stage I think we should prioritize user-related ones.
• This PR focuses on adding support to basic Pipeline functionalities.
• Note that add() code content is left commented for future work (if that would apply).

[xgrg]

Got it, thanks. That was worth asking the question.