Various improvements for Pipeline

[philippjfr]

Fixes:

• Ensure Pipeline._stage is correct after error (Incorrect Pipeline._stage after failed next stage #373)

New features:

• Implement ready_parameter on stages to block/unblock next button (Make Pipeline transition conditional on some value #613)
• Implement auto_advance to allow advancing automatically when ready (Control pipeline transitions from within stages #219)
• Implement Pipeline.define_graph for branching/converging workflows (Improve flexibility of Pipeline workflows #656)
• Implement new hv.Graph based subway map visualization
• Display error condition and show error/traceback by clicking on button (Pipeline error handling #163, Better control over exception handling in Pipelines #612)
• Make parameter inheritance configurable per stage (Make Pipeline.inherit_params controllable at the stage level #441)
• Disable panning on subway map (Disable pan tool on pipeline subway map #563)
• Make components of Pipeline individually composable

Others:

• Add tests
• Update docs

[philippjfr]

@sdc50 this is now ready for you to try. I've finally pushed the support for defining arbitrary graphs. The other work for routing parameters is still progressing.