Add initial structure to repository

[jorgensd]

• Adds a minimal package mypackage with a corresponding pyproject.toml file describing the package, and its dependencies for testing/documentation. This makes it pip-installable, i.e. pip3 install . from the root of the repository will install the package.
• Testing added in .github/workflows/test_package.yml
  • Shows how to setup scheduled runs (for instance running every day at 8am)
  • Shows how to use tools such as mypy and flake8 on the package.
• Shows how to set up a minimal testing environment on:
  • Ubuntu 22.04
  • Mac os 12
  • Windows server 2022
• Minimal script using pytest for testing code.

[jorgensd]

Resolves #2