Allow additon of new microscopy files as new positions to existing experiment folder

[Teranis]

Description:

In situations where users don't want to extract all microscopy files for all positions at the same time, it would be useful to be able to select a target experiment folder, and instead of just getting the options to either overwrite or add to existing positions, allow users to create new positions in the existing experiment folder.

[github-actions]

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[ElpadoCan]

Hi Timon, I just implemented this with PR #995

Let me know if it works, thanks!