Fix WcsNDMap and MapDataset cutout to support mode='partial'

[registerrier]

Description

This pull request solves issue #2754. First, an array of the correct size filled with 0 is created with the correct dtype. Then we add the content of the WcsNDMap using the the parenting cutout slices.
The cutout-slices are also used for the weight map that is used in WcsNDMap.

Note that I had put a test to ensure that other and weights have the same WcsGeom. This breaks many tests as it seems not to be the case in many situation. I found that puzzling. Maybe this should be clarified.

Dear reviewer

This is ready for review. We need to see whether mode='partial' should be the default for most observation loops. I tend to believe it should be since at least RingBackgroundMaker and FoVBrackgroundMaker need for field of view datasets to properly work.
Any opinion @adonath @AtreyeeS ?

I put v0.16 as milestone, and we'll see if we can really merge this in on time.

[adonath]

Thanks @registerrier, looks good to me. I have no further comments...

[adonath]

Thanks @registerrier, I'll try to add the geom check in master and see if it can be easily resolved. If not we will keep it like it is...

[adonath]

The "issue" is that the weights argument is sometimes provided as spatial map only (without energy axis). Internally we then rely on broadcasting to apply the weights. I think this behaviour is fine. In 81cdd1b I added at least a test for the spatial geometries...