Add prd (python generated directory) to PYTHONPATH

[ghost]

  "containerEnv": {
    "PYTHONPATH": "/workspaces/STIR2PETSIRD/PETSIRD/python"
  }

The aboce. should be added to the .devcontainer/devcontainer.json file.

Example implementation and use case.
https://github.com/ETSIhackers/STIR2PETSIRD/blob/52d21ad78533a2905766f16219ab4ad3caac2da4/.devcontainer/devcontainer.json#L126-L128

[KrisThielemans]

Problem is it needs the project-name. I think one of the .bashrc methods could be easier to, although even there people will have to rename presumably. (don't want to do an ugly find ~ -d -name PETSIRD).

I've created microsoft/yardl#108 which is probably the way forward.