Load initial settings from a YAML file

Loads settings from a YAML file. Two sample YAML files are provided.

Instructions:
- Click on the new tab "File".
- Press "Open...".
- Select either "Pair_of_particles.yaml" or "Two_stream_instability.yaml" and press "Open".

You can change the settings in the editor and press "Apply" to start with new initial conditions. If you like the new initial conditions, press "Save".