combine_by_coords could use allclose instead of equal to compare coordinates

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

When a coordinate in different dataset / netcdf files has slightly different values, combine_by_coords considers the coordinate are different and attempts a concatenation of the coordinates.

Concretely, I produce netcdf with (lat, lon, time) coordinates, annually. Apparently the lat is not the same in all the files (difference is 1e-14), which I suspect is due to different pyproj version used to produce the lon,lat grid. Reprocessing all the annual netcdf is not an option. When using open_mfdataset on these netcdf, the lat coordinate is concatenated which leads to a MemoryError in my case.

Describe the solution you'd like
Two options:

• add a coord_tolerance argument to xr.combine_by_coords and use np.allclose to compare the coordinates. In line 69 combine.py the comparison uses strict equality "if not all(index.equals(indexes[0]) for index in indexes[1:]):". This does not break the compatibility because coord_tolerance=0 should be the default.

• add an argument to explicity list the coordinates to NOT concatenate. I tried to play with the coords argument to solve my problem, but was not succesfull.

Describe alternatives you've considered

• I certainly could find a workaround for this specific case, but I often had issue with the magic in combine_by_coords, and imho adding more control by the user would be useful in general.

Additional context

Start in combine.py at the comparison around line 69 and trace how combine_by_coords decides whether coordinates can be concatenated. Reproduce the near-equal-coordinate case described for open_mfdataset, then compare the two proposed API approaches and their default behavior. Done means the chosen option handles tiny coordinate differences without changing strict-comparison behavior by default.