chore: make private fields readonly

[Tuditi]

Description of change

Changes the types of a class from private to readonly, so the dev experience of TS users increases.

Type of change

Choose a type of change, and delete any options that are not relevant.

• Enhancement (a non-breaking change which adds functionality)

How the change has been tested

Tested within firefly.

Change checklist

Tick the boxes that are relevant to your changes, and delete any items that are not.

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have checked that new and existing unit tests pass locally with my changes

[thibault-martinez]

(ignore the latest comment, GH bug)