• add plate boundary files from GSRMv2.1 and MORVEL56

• functions to read the boundary files and plot

Check here for the usage: https://github.com/yuankailiu/utils/blob/main/notebooks/PMM_plot.ipynb

Description of proposed changes

Reminders

• Pass Pre-commit check (green)
• Pass Codacy code review (green)
• Pass Circle CI test (green)
• Make sure that your code follows our style. Use the other functions/files as a basis.
• If modifying functionality, describe changes to function behavior and arguments in a comment below the function declaration.
• If adding new functionality, add a detailed description to the documentation and/or an example.