Event Camera X/Y coordinates

[steob92]

I notice we're using "Xoff/Yoff"

V2DL3/pyV2DL3/eventdisplay/fillEVENTS.py

Lines 110 to 111 in e3b59dd

	evt_dict["Xoff"] = DL3EventTree["Xoff"][mask]
	evt_dict["Yoff"] = DL3EventTree["Yoff"][mask]

For EVENTS GADF suggests the optional columns:

DETX type: float, unit: deg

        Reconstructed field of view X (see [Field of view](https://gamma-astro-data-formats.readthedocs.io/en/v0.3/general/coordinates.html#coords-fov)).

DETY type: float, unit: deg

        Reconstructed field of view Y (see [Field of view](https://gamma-astro-data-formats.readthedocs.io/en/v0.3/general/coordinates.html#coords-fov)).

I think this is what we're calling Xoff/Yoff?

For gammapy-background-analysis the camera X/Y is required when mimicing and scrambling data.

I've changed this in the 4f1159e. I think I caught everywhere on the ED side.

I don't see the camera X/Y stored in the vegas side of things. Is this something that can be easily implemented for vegas?