Add gridded POLDIRAD dataset

[observingClouds]

This PR adds the gridded POLDIRAD dataset available at https://observations.ipsl.fr/aeris/eurec4a-data/BARBADOS/POLDIRAD/gridded_data/PPI_PCP_L2GRID_v1/

To easy the access to the dataset:

• 4203 files have been referenced with kerchunk
• time dimension has been added based on the filename
• variables height, lat, lon, x_distance, y_distance have been assumed to be constant with time

The script used for the generation of the kerchunk file is referenced in the history of the kerchunk-dataset and is available here