Refactor RasterIndex internals (do not wrap PandasIndex)

[benbovy]

Closes #40.

This PR limits RasterIndex._wrapped_indexes to these two exact cases:

• {("x", "y"): CoordinateTransformIndex}
• {"x": AxisAffineTransformIndex, "y": AxisAffineTransformIndex}

This makes things much easier.

[benbovy]

Considering only these two cases, using a dictionary to store the wrapped indexes is overkill. This seems much cleaner:

class RasterIndex(Index):
    _index: CoordinateTransformIndex | tuple[AxisAffineTransformIndex, AxisAffineTransformIndex]

# or 

class RasterIndex(Index):
    _index: CoordinateTransformIndex | None
    _xy_indexes: tuple[AxisAffineTransformIndex, AxisAffineTransformIndex] | None

Storing coordinate names in RasterIndex._wrapped_indexes is also confusing and error-prone. We can retrieve the coordinate and dimension names of the X/Y axes from the CoordinateTransform objects wrapped by the underlying indexes.

[benbovy]

Ready for review!

[dcherian]

thanks! LGTM. w

[dcherian]

Merging to make progress on docs.