Currently can write variables from scalar Domains only (this is not checked!)

TODO
 - type conversion for attributes
 - read scalar domains
 - scalar variables in non-scalar domains
 - grids

Write out
    UnstructuredVectorGrid
    UnstructuredColumnGrid

    CellSpace variables at least for these two grids (one dimension 'cells')

Can now save/load
  PB.Grids.UnstructuredVectorGrid
  PB.Grids.UnstructuredColumnGrid

TODO
 - cartesian grids
 - add test cases to PALEOexamples
 - update doc

 Known issues / limitations:
 - PALEO currently has no concept of constant (time independent)
   variables. As a workaround, test in `variable_is_constant`
   looks for variable attribute :datatype = Float64 (and checks data is constant),
   where this attribute usually means constant but can also be used to
   label variables that won't be included in an AD Jacobian

- NamedDimension (used in UnstructuredColumnGrid and for Domain
  data_dims eg used for a wavelength grid) also includes FixedCoordinate
  variables, which may not be present in the output and may not even be
  uniquely names. As a workaround, for data_dims generate unique names and save/load an
  extra variable, for UnstructuredColumnGrid assume variable will be present
  (TODO this should be changed for consistency).

- Coordinate variables are not labelled with CF etc attributes

- UnstructuredColumnGrid uses a non-standard way of saving indices
  (similar to contiguous ragged array in the CF spec, but unlikely to
  be supported by other tools)

- can now save/load most grid types, except CartesianArrayGrid
- tests added to PALEOdev.jl
- Jupyter notebooks with Python xarray examples added to PALEOdev.jl