Scan: Multi layer sequence + retry

This PR is an attempt to solve tomo/ebs-tomo#96

Closes #4007 (closed)

It provides a new ScanSequence which is a real scan, so can be used with default scan features

• Add presets
• Set watchdogs
• Create and then run a sequence later
• So we can add a sequence to another sequence

def runner(context: SequenceContext):
    context.emit("my_channel", [1])
    s = loopscan(10, 0.1, run=False)
    context.add_and_run(s)
    context.emit("my_channel", [2, 3])

ch = AcquisitionChannel("my_channel", numpy.float32, ())

scan = ScanSequence(runner=runner, channels=[ch])

scan.add_preset(...)

scan.set_watchdog_callback(...)

scan.run()

The main difference with the implementation of bliss.scanning.group is that the grouping master is responsible of running the user function describing the sequence. This simplifies a lot the implementation.

In the user point of view, the API is not the same, and do not allow to add scans which was already started.