Smaller serialize

[valentinsulzer]

Description

Update serialization code to make the serialized models much smaller. Generally reduces file size by about 95%, e.g. from 6MB to 300kb for SPM

Fixes # (issue)

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #)

Important checks:

Please confirm the following before marking the PR as ready for review:

• No style issues: nox -s pre-commit
• All tests pass: nox -s tests
• The documentation builds: nox -s doctests
• Code is commented for hard-to-understand areas
• Tests added that prove fix is effective or that feature works