Partly revert of previous commits, with alternative solution to ensure
unicity of a Dataset's axes among its members.
Dataset is an OrderedDict instance object again, and its elements are
not any more a "Variable" object (intruduced a few commits ago). They
are just DimArray instances, as is most natural, but at each access via
__getitem__ the axes are copied from the Dataset.

This solves the problem mentioned in previous commit (metadata could
not be modified from "within" the dataset since only a copy was accessed)
and mimics a centralized axes attribute via access-copy of axes.

Remaining issue : axis names are used to related individual DimArray
elements and central axes. Any axis name change will break the Dataset.
dataset : bugfix, now copy also copies metadata
extended unit tests for dataset
add test_axes amnd test_dimarray, for now copy behaviour only
check for property in set_metadata
add dataset equality __eq__ method
can now rename all dimensions at once by assigning dims directly
setter for DimArray's labels property, similarly to dims
changed datasets structure : now all element in a Dataset
truely share their axes. When a new DimArray is inserted
in a Dataset, its axes are replaced with any pre-exising,
like-named axis from the Dataset. That way, any operation
on a Dataset axis is felt for all elements of the Dataset
which share that axis.

Note that a shallow copy (axes, metadata) is performed *prior* to inserting
the array in the data set.

Canonical __getitem__ method is used, without any further copy
when accessing a Dataset element. As a result, it is possible
to perform changes on single elements attributes from within
the dataset (e.g. metadata), which is more pythonic as far as
a dictionary is concerned.

An important consequence of this reference (instead of copy) access
is that after having been inserted in a Dataset, any operation on
a DimArray axes will further affect all other DimArray of the dataset
which share that axis.

If is of course possible to perform a copy of the axes (or of the whole
dimarray) if this behaviour is not desired.
add property setter for Datasets' dims
add various tests
updated tutorial to reflect changes in dataset