Add more helpful exception message when per-target data-array merge fails.

[leifdenby]

The current error message when merging data-arrays in https://github.com/mllam/mllam-data-prep/blob/main/mllam_data_prep/create_dataset.py#L114 could be more helpful by actually giving the sizes (coordinate names with sizes) of the data arrays being merged.

This could be achieved by adding a funciton like:

def print_dataarray_shapes_by_target(dataarrays_by_target: Mapping[str, list]) -> None:
    for target, dataarrays in dataarrays_by_target.items():
        print(f"{target}:")
        for i, da in enumerate(dataarrays, start=1):
            dims_str = ", ".join(f"{dim}={da.sizes[dim]}" for dim in da.dims)
            print(f"  {i}. {da.name}: {dims_str}")

And calling this to construct the following addition to the exception text:

$> print_dataarray_shapes_by_target(dataarrays_by_target)
state:
  1. pres_seasurface: state_feature=7, time=19, grid_index=3061036
  2. z: time=19, state_feature=48, grid_index=3061036
static:
  1. lsm: static_feature=2, grid_index=3061036
forcing:
  1. toa_radiation: forcing_feature=5, time=37, grid_index=3061036

Here it is for example clear that the forcing data-array is 37 time coordinate values where as state only has 19