[DEV] use pyhgtmap instead of phyghtmap for generating contour lines

[treee111]

This PR…

• switches to file-versioned and published (PyPi) python module pyhgtmap
• no manual (kind of difficult) installation of phyghtmap needed anymore
• closes Consider switching to pyhgtmap (fork) for contour lines #217

Considerations and implementations

Till now, there is no support for Windows, see agrenott/pyhgtmap#7

ToDO

adjust documentation
fix version of pyhgtmap and think of other publish-related stuff for this PR

How to test

1. create environment gdal-dev-pyh using env .yml file
2. switch to the environment
3. swith to this branch
4. Run wahooMapsCreator and include contour lines using -con
5. Check if the result is the same as using phyghtmap

Pull Request Checklist

• Reviewed the Contributing Guidelines
  • Especially the Pull-Requests section
• Commits (and commit-messages) are understandable
• Tested with macOS / Linux
• Tested with Windows

[alfh]

I think we can just use pyhgtmap with "jobs=1", then no forking is attempted, and it should work on windows as well. And then rather use asyncio to launch several instances of pyhgtmap. I'll do some testing on linux first and then on windows.