xarray adapter

[clbarnes]

Certain transformations can be applied to xarray, only operating on the coordinate vectors (possibly reordering them by transposing the main array).

These transform types could subclass a mixin, or the Transform could expose a method which may return None.

What should work:

• identity
• scale
• translation
• reflection
• mapAxis
• wrappers of the above:
  • sequence
  • bydimension
  • bijection