Add cube merging tool

[forman]

Is your feature request related to a problem? Please describe.

Currently, xcube gen can only handle one type of input. In many cases the multiple cubes generated from different input types must be combined into one single final cube.

Describe the solution you'd like

• CLI tool: xcube merge [-o|--output <output>] CUBE ...
• Python API function merge_cubes(*cubes: List[xr.Dataset]) -> xr.Dataset

Options will need to be added (later) to handle

• different spatio-temporal resolutions, coordinate units, and coverages of input cubes,
• merging of metadata,
• concatenating new observations (adding new time slices, refer to #218).

Additional context

For the DCS4COP project, we generate cubes from various inputs such as ocean colour products based on Sentinel-2 MSI, Sentinel 3 OLCI, and SEVIRI, observations as well as CMEMS SST.

In DCFS, reference and prediction results from machine learning need to be combined into a single cube.

See also #218.

[rabaneda]

Is this xcube merge tool also meant to merge 2 cubes next two each other into 1 cube? i.e. concatenating along dimensions lat, lon and time.