Xattree update

[wpbonelli]

• use new xattree builtin dict to array converter, remove custom impl (todo: support tabular data too... xattree should provide a single converter that supports either dict or table input)
• get rid of star syntax for sparse dict representations of arrays. this was more trouble than it was worth to generalize, and the caller can always use a comprehension to easily generate a still-pretty-sparse dict
• accommodate get_xatspec refactor (it returns XatSpec now, not the flattened chainmap)
• miscelleanous other cleanup and notes
• todo: xattree needs to distinguish array fill value from default. can't always interpret default unambiguously for optional arrays. e.g., when an optional array should be None (i.e. not present) by default, but should have a fill value when populated from a sparse dict or table representation