Load imports lazily

[hakonanes]

Description of the change

This PR introduces lazy imports of modules, following the specification in PEP 562.

The implementation in hyperspy/hyperspy#2850 helped a lot when figuring out a nice way to test lazy importing, thanks @ericpre!

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

Before this PR:

In [1]: %time import kikuchipy as kp
CPU times: user 7.65 s, sys: 556 ms, total: 8.21 s
Wall time: 7.54 s

After this PR:

In [1]: %time import kikuchipy as kp
CPU times: user 431 ms, sys: 164 ms, total: 596 ms
Wall time: 340 ms

Note that the import time of from kikuchipy.signals import EBSD does not change, as that import triggers all the imports in kikuchipy/signals/ebsd.py both before and after this PR.

For reviewers

• The PR title is short, concise, and will make sense 1 year later.
• New functions are imported in corresponding __init__.py.
• New features, API changes, and deprecations are mentioned in the unreleased
  section in CHANGELOG.rst.
• New contributors are added to release.py and .all-contributorsrc and the table
  is regenerated.