mclements

I have added some changes to address the recent R check notes and warnings: - I incremented the version number - To remove a note about R6 not having any...

Devin: pkgdown is complaining about the documentation for CohortCtstm in ctstm.R. I can't see the error - would you be able to help here, please?

Some commentary on the implementation: - I was not able to find another package on CRAN that allows LinkingTo: for ordinary differential equations in C++. I investigated adapting the [`rehuel`](https://github.com/Pakketeretet2/rehuel)...

I have now fixed the pkgdown issue.

@dincerti, I have now added some tests for this. This is now ready for review -- although admittedly this is a larger pull request than initially anticipated.

Devin: - I have now added an experimental cCTSTM implementation for semi-Markov models. - The implementation is similar to the Markov case, except that one needs to discretise for duration...

@dincerti Devin: this is now synced with main. Let me know if there is anything I can do to help. Sincerely, Mark.

Devin: I have also added some casts in a number of integer comparisons to address some long-standing warnings.