Allow loading of patterns from arbitrary scan group names in h5ebsd files

[hakonanes]

Description

Any HDF5 top group name for scan groups is allowed, not just "Scan 1", "Scan 2", etc., which is the case for v0.2.1. Addresses #184 (and fixes #185).

Checklist

• Docstrings for all functions
• Unit tests with pytest for all lines
• Clean style in as per black

Minimal example of the bug fix or new feature

If e.g. an EDAX TSL HDF5 file has the standard h5ebsd top groups "Manufacturer" and "Version" and three scans with group names "Scan 1", "Hello, C3PO!" and "Just the best patterns", all or some of the scans can be read into separate EBSD objects:

>>> import kikuchipy as kp
# Read multiple scans at once
>>> s1, s2, s3 = kp.load("my_file.h5", scan_group_names=["Scan 1", "Hello, C3PO!", "Just the best patterns")
# Read a single scan which is not the first scan in the file
>>> s_not_first = kp.load("my_file.h5", scan_group_names="Hello, C3PO!")
# Read the first scan in the file
>>> s = kp.load("my_file.h5")

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.

[coveralls]

Coverage remained the same at 100.0% when pulling eee788b on hakonanes:fix-h5ehsd-reader-top-group into d82a939 on kikuchipy:master.