Added convenience function for meas tool

[JeanLucPons]

This PR add 2 convenience functions to retrieve handle to measurement tool from callback.
It allows to add metadata in a more convenient way.
It also add a new event (INIT) when the scan starts.

ex (Add tune measurement at each orm measurement step):

from pyaml.common.constants import Action
from pyaml.accelerator import Accelerator
from pyaml.tuning_tools.measurement_tool import MeasurementTool
import numpy as np
import matplotlib.pyplot as plt

# Load the accelerator
sr = Accelerator.load("/home/esrf/pons/pyaml/examples/use_cases/config/bessy2/bessy2.yaml")

# ORM callback
def orbit_callback(action: int, cdata):

    if action == Action.INIT:

        MeasurementTool.get_from_cb(cdata).latest_measurement["tune"] = []

    elif action == Action.MEASURE:

        # Add additionnal data to the scan
        tune = MeasurementTool.get_peer_from_cb(cdata).tune.readback()
        MeasurementTool.get_from_cb(cdata).latest_measurement["tune"].append(tune.tolist())

    return True


# Start scan
sr.design.orm.measure(callback=orbit_callback)
# Save measurement data including additionnal metadata
sr.design.orm.save("mat.json")