improved examples and test, broken
intermediate commit - save progress
put netCDF default write format in dimarray.config
work in progress:
- prettyprinting.py : concentrate various representations functions
- various classes broken down, in view of unifying DimArray and
  DimArrayOnDisk, Dataset and DatasetOnDisk
- unique Indexable class that handle the tricky indexing part
- now the functions read_nc and so one are simply wrapper around the
  classes
- major changes:
    - DimArray, Dataset, Axis derive from the same abstract class
      whether in-memory or on-disk
    - unified indexing
    - greatly simplified and sped-up indexing and reindex_axis using numpy
    searchsorted
    - metadata now stored in an `attrs` attribute, while still
      supporting `.` syntax
- Still much debuggin to do, run tests and so on