Load imports lazily

[hakonanes]

Description of the change

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

The implementation in https://github.com/hyperspy/hyperspy/pull/2850 helped a lot when figuring out a nice way to test lazy importing, thanks @ericpre!

Progress of the PR

• [x] Docstrings for all functions
• [x] Unit tests with pytest for all lines
• [x] 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.