Revert wrong merge of psyplot-plugin-template

with !27 (merged) I accidently opened an MR against the wrong repo (it should be against the psyplot-plugin-template repo). So I am reverting it in !28 (merged). However, for the purpose of a clean git history, I will force-push to the main branch once !28 (merged) is merged.

• !28 (merged) merged
• force-pushed to main to get back to the state prior to !27 (merged)
• reimplement the changes from !26 (merged) (see !29 (merged))