[uss_qualifier/resource] Add base class for geospatial modifier

[the-glu]

Continuing on #1465 for #1458, this PR introduce:

• GeospatialResource, as suggested to ease implementation of
• GeospatialModifier, allowing a resource to generate unique 'Geospatial' resources

Include simple unit tests with a TestSquareResource.

[BenjaminPelletier]

I think I misread #1465 and would like to adjust it before looking closely at this one. The issue is that I would expect "resource modifier" to mean something that modifies resources, but the more important aspect of ResourceModifier in #1465 is that it presents as the modified resource itself, so it seems like a better name would be "IndexAdjustableResource". That's important because in this PR we have GeospatialModifierResource and with a name like that, I would expect it to handle any geospatial modification we might want to make to resources. Instead, it's actually just one particular way to offset GeospatialResources. With AdjustableResource, I think it would be more obvious that a name like GeospatiallyOffsetResource is probably more descriptive than GeospatialModifierResource. I think this is important because this is an external surface of the product that users will directly see and touch. I'll propose an adjustment PR.

[the-glu]

I think I misread #1465 and would like to adjust it before looking closely at this one. The issue is that I would expect "resource modifier" to mean something that modifies resources, but the more important aspect of ResourceModifier in #1465 is that it presents as the modified resource itself, so it seems like a better name would be "IndexAdjustableResource".

Ok, to do that (behaving as the modified resource, witch it doesn't do for now) we can just have it expose the underlying type as well + proxy with getattr()/setattr() attributes to the underling resources.

Let me know, I can also do the change :)