Add a meta to hold current process of plot merge

[TheMeinerLP]

Overview

Fixes #3056

Description

Fixes the current duping problem when plots are merged in survival.

Submitter Checklist

• Make sure you are opening from a topic branch (/feature/fix/docs/ branch (right side)) and not your main branch.
• Ensure that the pull request title represents the desired changelog entry.
• New public fields and methods are annotated with @since TODO.
• I read and followed the contribution guidelines.

[PierreSchwang]

Should be enough to just return this.getMeta("merging") != null I guess?

[TheMeinerLP]

Maybe 😄

[dordsor21]

Do we not have a hasMeta ?

[TheMeinerLP]

Nope

[PierreSchwang]

I guess that Meta-System could use some refactoring either way. A hasMetadata extension would be great, as well updating the backing Map to an EnumMap and have enum constants for each metadata key. I guess that would benefit for maintainability and documentation purposes.

[TheMeinerLP]

That would be great. Lets coordinate that together I will help you out.

[TheMeinerLP]

This PR needs be tested. @OneLiteFeather and me have tested locally and sometimes was working and sometimes not. I am now out for today.
I still have some improvements on my pc, but I am not sure if there are working. The cause why I am not pushing it, is the locally testing are so buggy. Sometimes the breakpoints are not holding in the right method/line.