fix: mark get/set in AbstractMatrix as abstract

[Dimava]

While I was implementing a BandMatrix for myself I've noticed that editor does not suggest me to implement get/set fields which I knew were abstract.

Here's a fix.

Please check if it breaks any typings by making classes not implementing get/set abstract (those classes should be made abstract then)

[Dimava]

Seems like I've missed the fact that types are provided and not compiled

[codecov[bot]]

Codecov Report

Base: 66.67% // Head: 66.67% // No change to project coverage :thumbsup:

Coverage data is based on head (536f08d) compared to base (5ec6888). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #155 +/- ## ======================================= Coverage 66.67% 66.67% ======================================= Files 32 32 Lines 3232 3232 Branches 514 514 ======================================= Hits 2155 2155 Misses 997 997 Partials 80 80 ``` Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mljs). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mljs)

:umbrella: View full report at Codecov.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.