Write/read support for EBSD signal with 0/1 navigation dimension in kikuchipy's h5ebsd format

[hakonanes]

Description of the change

Write/read support for EBSD signal with 0/1 navigation dimension in kikuchipy's h5ebsd format.

Close #269.

Progress of the PR

• Docstrings for all functions
• Unit tests with pytest for all lines
• Clean code style by running black via pre-commit

Minimal example of the bug fix or new feature

>>> import kikuchipy as kp
>>> s = kp.data.nickel_ebsd_small()
>>> s
<EBSD, title: , (3, 3|60, 60)>
>>> s.inav[0].save("filename.h5")
>>> s2 = kp.load("filename.h5")
>>> s2
<EBSD, title: , (3|60, 60)>

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  the unreleased section in doc/changelog.rst.

[hakonanes]

Will merge when checks pass.