Update of the dictionaries that are used to build the MIVOT_INSTANCE

[lmichel]

The model references in the dictionaries that are used to build MivotInstance objects are more consistent:

• Both dmtypeand dmrole keys are set with complete VODML-ID (model:a.b.c).
• Dictionary keys derived from dmrole are truncated (model:a.b.c-> c). The names of Python attributes are equals to these keys.

    {
        "dmtype": "mango:EpochPosition",
        "longitude": {"value": 359.94372764, "unit": "deg"},
        "latitude": {"value": -0.28005255, "unit": "deg"},
        "pmLongitude": {"value": -5.14, "unit": "mas/yr"},
        "pmLatitude": {"value": -25.43, "unit": "mas/yr"},
        "epoch": {"value": 1991.25, "unit": "year"},
        "coordSys": {
            "dmtype": "coords:SpaceSys",
            "dmid": "ICRS",
            "dmrole": "coords:Coordinate.coordSys",
            "spaceRefFrame": {"value": "ICRS"},
        },
    }

This makes the understanding of these dictionaries and the use of the MivotInstance objects quite easier and more intuitive.

[bsipocz]

I'll rebase this to fix the changelog conflict, and will go ahead and merge.

[bsipocz]

Thank you @lmichel!