Warn users about upcoming move to UNC draft v0.2 #40

obsarray currently follows v0.1 of the UNC specification, but will move to draft v0.2 in future, which is proposed for inclusion in the CF conventions and isn't backwards compatible with v0.1.

Should warn users that this change is coming before it happens - e.g. a FutureWarning the first time uncertainty metadata is written in a session, with an option to explicitly choose v0.1 to silence it, plus a page in the docs explaining what's changing.