Eliminate infrequently used doxygen commenting style

[ischoegl]

Changes proposed in this pull request

C++ doxygen documentation allows for several commenting styles. While other C and C++ styles are frequently used in the Cantera code base, the variant /// is used infrequently. In order to improve consistency, this PR eliminates /// in favor of the //! commenting variant.

I also noticed that /*! is much more frequently used than /**, but replacing the latter would be somewhat more intrusive.

Checklist

• The pull request includes a clear description of this code change
• Commit messages have short titles and reference relevant issues
• Build passes (scons build & scons test) and unit tests address code coverage
• Style & formatting of contributed code follows contributing guidelines
• The pull request is ready for review

[ischoegl]

Withdrawing - this is likely unnecessary churn.

[speth]

I was going to merge this as soon as I'd had a chance to skim through it.

[ischoegl]

@speth ... feel free to reopen and merge - the branch is still active.

[speth]

Simple, and worth the little bit of extra consistency that helps identify what is and isn't a Doxygen comment.