Update examples and add new examples

[gboeing]

This PR:

• resolves Add Clustering Example by Network Distance #58 by adding network-constrained clustering example notebook 18
• resolves Add example for OR condition in custom filter #90 by adding a custom_filter union example to notebook 08
• resolves Add example demonstrating new buffer_geometry helper function #92 by demonstrating buffer_geometry function in notebook 03
• resolves Add example of attaching features to nearest nodes #83 by adding example of attaching features to nearest nodes in notebook 16
• moves away from old v1 API to instead call functions like ox.module.function()
• fixes GeoPandas unary_union deprecation warnings