Volatility for Glicko2Rating::unrated() should be specified

[halvnykterist]

Hi!

I'm using the library to calculate ratings for http://ratingupdate.info/, and was running into an issue with deviations increasing quite rapidly over time, despite the system constant being very low. After debugging a bit, I came to the conclusion that new glicko2 ratings had a volatility of 0.06 assigned was the problem.

In the original paper, the text is:

Set the player’s volatility to 0.06 (this value depends on the particular application).

This does mean that From<GlickoRating> on Glicko2Rating isn't really something that can be implemented, which has implications for the rest of the API.

[DenialAdams]

Thank you for the feedback, sorry that you had trouble!

I'll look into this today - I guess at a bare minimum we should create a method Glicko2Rating::from_glicko_rating_with_volaitility, and then we can decide if we want to also remove the From impl.

[halvnykterist]

Yeah, that sounds about right. I think a better "uninitialized" constructor would be good, too.