Updates publish to TestPyPI GitHub Action / Adds publish to PyPI GitHub Action

[tomaroberts]

Resolves #24 and improves upon implementation of #23

• Implements dunamai
• Version number updated with each commit using X.Y.Z.postN format (SemVer, so compatible with PyPI)
• Publication on TestPyPI with every push on main
  • Verifies that python package will build and runs basic nii2dcm CLI check
• Publication on PyPI with every Release
  • Version number set by Release tag in format vX.Y.Z