Add 3 examples of usage pyvisgraph

[tjansson60]

Examples folder

A initial set of examples using the pyvisgraph package and related packages to build, visualize and calculate distances based on the GSHHS shoreline files.

• build_graphfile_from_shapefiles.py - Build a graphfile using the shape files of the shorelines from GSHHS.
• calculate_shortest_distance.py - calculate the shortest distance between two points and a graphfile.
• plot_shortest_path_on_map.py - find the shortest distance between two points and plot them on an interactive map (using folium). Save the plot as a html file.

[TaipanRex]

Merged, I did make some changes which are mentioned in the commits.

Thanks for the PR!