Add a check for unique IDs

[laurenmock]

I'm working with a dataset with several rows per individual, and I was able to run estimate_gps with non-unique IDs. It would be helpful to confirm that the "id" columns in w and c contain only unique values.

[Naeemkh]

Hi @laurenmock, thanks for opening the issue. Please fork the package in your own Github account, make your changes, and submit a PR.
Please see the following page for contribution details: https://nsaph-software.github.io/CausalGPS/articles/Developers-Guide.html
Please let me know if you have any questions.