Minor notebook fixes for Fiona and GeoPandas

[geographika]

Switch to unary_union for Shapely as cascaded_union is deprecated and shows warnings:

ShapelyDeprecationWarning: The 'cascaded_union()' function is deprecated. Use 'unary_union()' instead.
  dst_poly = cascaded_union(nboxes)

Use geoms for multipart feature access in Shapely to avoid the following warnings:

ShapelyDeprecationWarning: Iteration over multi-part geometries is deprecated and will be removed in Shapely 2.0. Use the `geoms` property 
to access the constituent parts of a multi-part geometry.