Improvement of MeasurementTool interface

[JeanLucPons]

This PR improves doc and interface of MeasurementTool to allow free metadata to be added in MeasurementData.

# Callback example that adds free metadata in the MeasurementTool data.

def callback(action: Action, data:dict):
    src = data["source"]
    # Add metadata
    if isinstance(src,ChromaticityResponseMatrix) and action==Action.MEASURE:
        if "MyData" not in src.get():
            src.get()["MyData"] = []
        src.get()["MyData"].append(f"MyValue:{data["idx"]}") # Add a dummy value (the index here)
    return True