Howto: add a new stage

Adding a new stage implies the addition of the stage folder to the cmp/stages directory and according modification of the parent pipeline along with insertion of a new image. Copy-paste of existing stage (such as segmentation stage) is recommended. Required members and methods for stages are :

• Members: name => string, defining the name of the stage. config => instanciation of a HasTraits class. inputs, outputs = arrays of strings, that will be used to create the inputnode and outputnode.
• Methods: create_workflow => method adding nodes to the stage's workflow. define_inspect_outputs => called to fill the inspect_outputs and inspect_outputs_dict members. has_run => returns True or False.