Refactor and make a SavedMatrix immutable

[jan-goral]

Author the user story for this feature

As a developer, I want to have SavedMatrix class immutable like a rest of Flank so I can easier understand and operate on flank implementation.

Is your feature request related to a problem? Please describe.
This is last unfinished part of #718. The SavedMatrix is written in object oriented style. Most of flank object oriented code was already rewritten to functional style, so SavedMatrix shouldn't be exception.

Definition of done

• SavedMatrix is rewritten to immutable data class.
• All methods from SavedMatrix are converted to functions (or extension functions).
• All implementation should work as well, without any regression.

Additional context
This is low priority refactor issue

[Sloox]

On hold due to a large pull request that is more important and will bring some changes to the class: #919