Move conversion functions from highlevel to ease

[kodebach]

Basics

• Short descriptions should be in the release notes (added as entry in
  doc/news/_preparation_next_release.md which contains _(my name)_)
  Please always add something to the the release notes.
• Longer descriptions should be in documentation or in design decisions.
• Describe details of how you changed the code in commit messages
  (first line should have module: short statement syntax)
• References to issues, e.g. close #X, should be in the commit messages.

Checklist

• I added unit tests
• I ran all tests locally and everything went fine
• affected documentation is fixed
• I added code comments, logging, and assertions (see Coding Guidelines)
• meta data is updated (e.g. README.md of plugins and METADATA.ini)

Review

• Code is conforming to our Coding Guidelines
• APIs are conforming to our Design Guidelines
• Code is consistent to our Design Decisions
• Documentation is concise and good to read
• Examples are well chosen and understandable

[markus2330]

Thank you for creating this PR! Great idea to move them.

[kodebach]

@markus IMO this is ready to merge, link-checker won't work (see #2915 (comment))

[markus2330]

Thank you! I'll merge soon (I wait for the master build to complete)