RDP filter

[avitase]

Would you accept a pull request that adds an implementation of the RDP algorithm?

An implementation that uses the shortest distance between a point and a geodesic line segment (or alternatively a Rhumb line segment?) is relatively straightforward, but there are some small caveats; hence, having a robust implementation here can save others some time scratching their head in a debugger session. However, I think this feature could come in handy in libraries such as pyproj that wrap PROJ but somehow it feels more natural to implement this feature once here instead of multiple times in different wrappers. What do you think?