Initial commit of AlvinXY module

[quietlychris]

I recently put together a Rust implementation of a function that converts between a local and global coordinates (I believe in a NED direction scheme?), and rather than build a new crate for it, thought it might make more sense to add it to this one as a submodule.

This primarily takes place with two public functions (xy2latlon and latlon2xy) which do the conversion based on a provided origin and local or GPS coordinate. There's basic test coverage I used to verify it was working, and this implementation was used to to build a mission file for an AUV mission earlier this month, which was successful.

I don't have much experience with contributing to other people's projects, so if there's changes that you'd like made to get things up standard, just let me know!