FIX: convert each Py-ART Grid only once in get_dd_wind_field

[Wahaj18]

Closes #167

get_dd_wind_field converted a list of Py-ART Grids inside a redundant outer loop:

for x in Grids:
    new_grids = [read_from_pyart_grid(x) for x in Grids]

so every grid went through read_from_pyart_grid once per grid in the list. That conversion pops data from the Grid's radar_latitude/radar_longitude/radar_altitude dicts, so the second pass over the first grid fails with KeyError: 'data'. In practice, passing two or more Py-ART Grids straight to get_dd_wind_field always failed. Lists of xarray Datasets and a single Py-ART Grid were not affected.

This PR drops the outer loop so each grid is converted once.

Testing: added test_list_of_pyart_grids, which passes two Py-ART test grids to get_dd_wind_field. It fails on main with KeyError: 'data' and passes with this change. The rest of test_cost_functions.py, test_initialization.py and test_retrieval.py passes locally on the SciPy engine (Jax/TensorFlow tests skipped). I left out test_get_iem_data (network) and test_twpice_case_parallel (dask), and the two HRRR tests fail only because cfgrib isn't installed in my environment. ruff and black are clean on the changed files.