feat[next]: Embedded support for skip value connectivities

[havogt]

• Introduces common._DEFAULT_SKIP_VALUE = -1, currently we don't support execution with skip_values which are different than that default.
• inverse_image works by ignoring the skip_value for the inverse image calculation. current result is the smallest hypercube that contains all non skip values that are requested.

Additional changes:

• Adds a ffront test for the fvm nabla example
• Fixes a lowering bug from past to itir, where domain didn't respect order of horizontal and vertical dimension

[egparedes]

It looks correct to me. My comments are mostly about style, and some questions and typo fixes.

[egparedes]

Last questions and comments...

[egparedes]

Out of scope of this PR but just to not forget: it doesn't make sense to use a property for plain attribute access just because a property was defined in a parent class. We could add a simple utility to eve to disable parent properties by overwriting them with a non-data descriptor like this:

def disable_property(default=...) -> functools.cached_property:
    return functools.cached_property(lambda _: default)

then this should work:

Suggested change

	_codomain: common.DimT
	_skip_value: Optional[core_defs.IntegralScalar]
	codomain: common.DimT = disable_property()
	skip_value: Optional[core_defs.IntegralScalar] = disable_property()

Inside a dataclass, it would also work to define a field with a default like this:

    codomain: common.DimT = dataclasses.field(default=...)

although this would change the semantics by making codomain optional, where disable_property wouldn't affect the behavior of the dataclass at all.

[egparedes]

ILGTM.