Packaging for packages.octave.org

[cbm755]

In the octave_pkg/ dir is a script that makes the package file.

The .tar.gz just needs to be put somewhere and linked to the yaml file, drafted here: gnu-octave/packages#615

We could use the GitHub Releases...

• chebfun is currently at v5.7.0
• so a reasonable semantic versioning describing its current stay is 5.7.1-dev0
• we could release as 5.7.1-dev0.oct1 which I think is a valid semantic version: https://semver.org/
  • but I don't know if Octave's pkg is going to handle such...
  • possibly yes, if we leave id: "dev" in the yaml file...