Implement gnubg position ids

[carsten-wenderdel]

To store rollout data efficiently, it makes sense to use an established format like GnuBG position ids.

There are two subtasks, one for reading and one for writing. The signatures of the functions could look like this:

pub fn from_id(id: &str) -> Position {...}

and

pub fn to_id(&Position) -> String {...}

[carsten-wenderdel]

Solved with PR #11