This adds a lot of warnings; DimensionGraph usage will be migrated in
later commits.

Due to those warnings, one test in test_obscore.py has been
temporarily disabled, because it counts warnings, and I couldn't
figure out how to make it only count the kind of warning it's actually
looking for.

The confusingly-named DimensionUniverse.getStatic* methods are no
longer needed after this, but while I think the middleware team has
informally agreed we should drop them and go back to attributes, I
don't think that's been RFC'd, so they stay for now.

The new skypix attributes help with the other goal here: using
set-membership tests rather than isinstance checks in places where
we care about different types of dimensions.  This will work better
when DimensionElement objects appear more rarely, in favor of str
names.

This includes replacing '.graph' with '.dimensions' (to go along with
replacing DimensionGraph with DimensionGroup), adding '.mapping' and
'.required' to replace '.full', and deprecating Dimension-instance
lookup and iteration (which effectively deprecates the Mapping
interface).

DatasetType.dimensions needs to continue to return DimensionGraph
during the deprecation period, since that's the name we want to use
long-term (unlike e.g. DataCoordinate.graph).  That means we rely on
the fact that we've also deprecated all of the things DimensionGraph
can do that DimensionGroup can't do (like iteration), and we rely on
those warnings instead of making DatasetType.dimensions itself warn.
That should let a lot of usage just blindly pass
DatasetType.dimensions to something else and not care about what type
it is throughout the deprecation period.

Only usage was in DimensionGraph.

The old DataCoordinate.makeEmpty is not being deprecated because that
wasn't included on RFC-834 and we don't have a good reason to get rid
it, but we've now got a consistent suite of DataCoordinate factory
methods with from_full_values and from_required_values (whose camelCase
forms _did_ have a good reason to be retired, as this aided with the
transition from DimensionGraph to DimensionGroup).