Various tidying.

- Aim for PEP-8.
- Change getFieldValue signature to keyword parameters: allow omitting
  defaults; allow documenting parameters (``creation=True`` is readable,
  but just ``True`` is opaque).
- Improve docstrings.
Tidy. Consistent indentation, naming.
Tidying docstrings, use string interpolation for HTML.
Don't use tabs.
Indent 4 spaces.
Tidying. Some docs & comments, some code.
Minor tidying.
Merge branch 'github-main' of github.com:plomino/Plomino into jean-cleanup
Tiny docstring tweaks
Fix: editmode is unused.
Formatting, spelling ..
Whitespace changes.
merge