Generate H3 hexgrid shapefiles

Generating H3 hexgrid shapefiles

```python

# Imports
from h3 import h3
import folium
from shapely import geometry
import fiona
import geopandas as gpd
import descartes

# functions
def get_subindexes(h3_key):
    return h3.h3_to_children(h3_key)


def flatten(indexes):
    return [item for sublist in indexes for item in sublist]


def get_indexes_for_level(depth, current_level=0, indexes=h3.get_res0_indexes()):    
    print(f'Generating cells for depth: {depth}, current_level: {current_level}, idx_len: {len(indexes)}')
    if depth == current_level:
        return indexes

    indexes = flatten([get_subindexes(index) for index in indexes])
    current_level+=1
    return get_indexes_for_level(depth, current_level, indexes)


def generate_shapefile(level):
    indexes = get_indexes_for_level(level)
    # Define Schema
    schema = {
        'geometry': 'Polygon',
        'properties': {
            'id': 'int',
            'h3_key': 'str',
        },
    }

    # Write to Shapefile
    file_name = f'./shapefiles/h3_level_{level}.shp'

    print(f'Writing cells:{len(indexes)}, level: {level}, file: \'{file_name}\'')
    
    with fiona.open(file_name, 'w', 'ESRI Shapefile', schema) as c:
        
        for id, h3_key in enumerate(indexes):
            polygon = geometry.Polygon(h3.h3_to_geo_boundary(h3_key, True))
            c.write({
                'geometry': geometry.mapping(polygon),
                'properties': {
                    'id': id, 
                    'h3_key': h3_key
                },
            })

    print(f'DONE')


# Run

level = 2

generate_shapefile(level)
shape=gpd.read_file(f'./shapefiles/h3_level_{level}.shp')
shape.plot(figsize=(30,30),cmap="Blues")
```