Improve doxygen documentation of composite objects

[ischoegl]

Changes proposed in this pull request

Create new grouping to provide for easier overview of C++ Solution, Interface and SolutionArray.

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

[speth]

Thanks, @ischoegl, this looks good to me.