Add support for MERRA2 data

[jacobbieker]

To use MERRA2, we need to make some updates to datapipes. Primarily:

1. Add a function in load_nwp that can load the MERRA2 Zarr and transform it into the standard NWP format that datapipes uses, which means, 5 coordinate variables
   init_time_utc, step, latitude, longitude, channel
2. Add test data and a test to show that opening the data gives the correct shape

Example shape from loading ECMWF:

<xarray.DataArray 'ECMWF_BLAH' (init_time_utc: 1, step: 2, channel: 2,
                                latitude: 221, longitude: 221)> Size: 781kB
dask.array<transpose, shape=(1, 2, 2, 221, 221), dtype=float32, chunksize=(1, 2, 2, 221, 221), chunktype=numpy.ndarray>
Coordinates:
  * init_time_utc  (init_time_utc) datetime64[ns] 8B 2023-09-25T12:00:00
  * latitude       (latitude) float64 2kB 31.0 30.95 30.9 ... 20.1 20.05 20.0
  * longitude      (longitude) float64 2kB 68.0 68.05 68.1 ... 78.9 78.95 79.0
  * step           (step) timedelta64[ns] 16B 00:00:00 01:00:00
  * channel        (channel) <U5 40B 'dlwrf' 'dswrf'
Attributes:
    Conventions:             CF-1.7
    GRIB_centre:             ecmf
    GRIB_centreDescription:  European Centre for Medium-Range Weather Forecasts
    GRIB_subCentre:          0
    institution:             European Centre for Medium-Range Weather Forecasts

[AUdaltsova]

Fixed with #310