Make the mhs/common/definitions module an actual module

[adrianclay]

Why

The init.py file is required to make something a package.

See also: https://docs.python.org/3/tutorial/modules.html#packages

Type of change

Internal change (non-breaking change with no effect on the functionality affecting end users)

Checklist:

• I have performed a self-review of my code
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have updated the Changelog with details of my change in the UNRELEASED section if this change will affect end users