Bugfix: Undefined variable in POIExt

[danareu]

This PR fixes two bugs in POIExt that occured during the previous PR:

• Fix undefined variable 𝒽 and use 𝒽₀ instead
• Define op_per_strat variable as previously undefined

[JulStraus]

The bugfix is related to #9 and will close #10. There is one thing you must fix before we can merge it: Update the version number in the Project.toml file as well.

[danareu]

@JulStraus Update the version number in Project.toml

[JulStraus]

LGTM. I will merge once the CI ran through