Update library to v2.4 feature parity

[AgileJoshua]

This repository is a dotnet/csharp implementation of https://github.com/geographiclib/geographiclib/

Tags track the current feature parity version of this library compared to the c++ version of the library.

Your task is to update the current csharp version to match feature parity with the root repository tag v2.4. Our current version matches v2.3 tag in the root repository.

Identify changes that are needed and then implement them fully. Releases are managed manually, so do not attempt to create nuget packages or similar release artifacts. Once completed tag the last commit here with v2.4 so we can identify the correct source code.