Carving: Handle case when opLabeling.NonzeroBlocks isn't ready yet.
OpCarving: Fix null pointer crash if project is deserialized without an mst.
Fixed __init__ args for several workflows.
Shell, ProjectManager, ilastik.py changes:
ilastik.py: Added --new_project arg
Shell: Added ShellABC to define the common interface shared by IlastikShell and HeadlessShell
Shell: ProjectManager is responsible for mapping workflow name to workflow class.
HeadlessShell: Support workflow_cmdline_args
ProjectManager: Call Workflow.onProjectLoaded() after project is loaded.