Port msd to pure python

[DomFijan]

As part of the nanobind port msd submodule must be ported to pure python.

Description

msd submodule has been ported to pure python.

Motivation and Context

As part of the nanobind port msd submodule must be ported to pure python.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds or improves functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation improvement (updates to user guides, docstrings, or developer docs)

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• I have updated the documentation (if relevant).
• I have added tests that cover my changes (if relevant).
• All new and existing tests passed.
• I have updated the credits.
• I have updated the Changelog.

[joaander]

Thanks!