Meta 2

[stefraynaud]

Description

This is a major pull-request.

In this PR:

• xoa.cf module is moved to xoa.meta and splitted it in submodules for better maintaining.
• Pure numeric modules and functions are move to the now xoa.core module, like interp, num and others. This intent is to clearly separate very specific numerical computations from the xarray interface.
• Tests are moved outside the library to the tests/ directory.
• Data samples are removed from the library and managed remotely with pooch.

Check list

• Tests added
• User visible changes (including notable bug fixes) are documented in CHANGES.rst
• New modules are listed in api.rst