interpolate_na

[keewis]

interpolate_na is difficult to get to work without compatibility code (np.interp would need to gain support for axis and the scipy interpolation functions would need to start supporting duckarrays), so this adds a wrapper.

• towards unit-aware versions of unsupported functions #70
• Tests added
• Passes pre-commit run --all-files
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst