Support additional axes in SkyModel.evaluate_geom(geom)

[registerrier]

Is your feature request related to a problem? Please describe.
Currently, SkyModel.evaluate_geom(geom) does not support axes that are not energy_true. Calling it on a Geom object with additional axes will return an array with incorrect shape.

Describe the solution you'd like
SkyModel.evaluate_geom(geom) should return an array of the correct size copying the contents for the additional axes.

Special caution should be taken for time axis. For three reasons:

• currently the time is evaluated via the gti argument. This is an incorrect approach as gti is relevant on the MapEvaluator, not on the model. Note that a TimeMapAxis.from_gti() factory method exists.
• evaluate_geom actually integrates over time. Is this still necessary? Does it make sense?
• A MapAxis can represent a time-like quantity but does not contain a reference time. This has to be verified.

Another question, is how to handle energy axis (instead of energy_true).

[registerrier]

This still needs to integrate models using a time axis rather than GTIs.

[Astro-Kirsty]

This should be closed by #4828 ?

[AtreyeeS]

The question of energy vs energy_true in models remains unsolved. It can be discussed in a separate issue