Implement UNC draft v0.2 conventions #41

Add support for draft v0.2 of the UNC specification, which should ultimately become the standard through the CF conventions. The main changes from v0.1 are summarised in Appendix B of the spec.

Idea is to add a backend system that handles interpreting the conventions, with a backend per version - v0.2 becoming the default, and v0.1 still available through obsarray.set_options(unc_conventions="v0.1") (see #40). Since the v0.1 and v0.2 attribute names don't overlap, it should also be possible to auto-detect which version a dataset follows when reading it.

Switching the default is a breaking change for anyone writing files with obsarray, so probably one for a major release.