It was previously not possible to cleanly access the underlying data of
a `StokesSpectralCube`, which only had the `_stokes_data` method. This
commit implements a simple `StokesSpectralCube.stokes_data` method for
public access.

For some reason, reshaping the array then running map_blocks leads to memory issues when dealing with Numpy memory mapped arrays, e.g. with FITS files. This new approach avoids reshaping the array at all.

Avoid memory issues when computing statistics

…endian data and provides no performance improvements

Don't use bottleneck in statistics()

As per the second suggestion in radio-astro-tools#750 - the Stokes data can now be
accessed via cube['I'], cube['Q'] etc.

Assigning new keys is not yet supported and will throw
NotImplementedError if attempted.

Remove if statement that stops tox from running tests

It was previously not possible to cleanly access the underlying data of
a `StokesSpectralCube`, which only had the `_stokes_data` method. This
commit implements a simple `StokesSpectralCube.stokes_data` method for
public access.

As per the second suggestion in radio-astro-tools#750 - the Stokes data can now be
accessed via cube['I'], cube['Q'] etc.

Assigning new keys is not yet supported and will throw
NotImplementedError if attempted.

… stokes_methods