Add operation data_frame_find_closest()

[forman]

Expected behavior

Users of Glaciers CCI data require an operation that would provide the Glaciers that are closest to a given location. With the result, they would then perform other operations, e.g. finding the largest glacier within the new Glacier set.

Proposed signature is:

data_frame_find_closest(gdf: gpd.GeoDataFrame, 
                        location: GeometryLike, 
                        max_results: int = 1,
                        distance_var_name: str = None) -> gpd.GeoDataFrame

If distance_var_name is given, a column is included that contains the actual great-circle distances in meters.

Actual behavior

Such operation does not exist yet.

Specifications

Cate 1.0 ... 2.0.dev15