Support `merge_cubes` for vectorcubes

[VincentVerelst]

Currently, the merge_cubes process only works for rastercubes. Can this process also be made available for vectorcubes? This would be needed for the WorldCereal project.

[soxofaan]

what would the initial use case be here? appending cubes along geometry dimension? appending along bands dimension? should there be support for overlap resolving?

[VincentVerelst]

• The vectorcubes to merge will have no time dimension
• In theory all vectorcubes will have the same bands, only their geometries will be different, so this mean the appending of the cubes should be along the geometry dimension.
• An overlap resolver might be needed for two identical geometries
• In terms of Pandas DataFrames, it should behave similar to cube = pd.concat([cube_one, cube_two], ignore_index=True)

Here is a visual example of what the merging should look like (including the unlikely case that the two cubes don't have all the same bands):

First cube:	Band_1	Band_2	geometry
1	'A'	POINT (0,0)
2	'B'	POINT (0,1)

Second cube:	Band_1	Band_3	geometry
3	'C'	POINT (1,1)
4	'D'	POINT (1,2)

Merged cube:

Band_1	Band_2	Band_3	geometry
1	'A'	NaN	POINT (0,0)
2	'B'	NaN	POINT (0,1)
3	NaN	'C'	POINT (1,1)
4	NaN	'D'	POINT (1,2)