Preserve comments in files

[gassmoeller]

Ideally we would like to preserve user comments in parameter files, but this needs a number of changes in different projects:

• dealii::ParameterHandler needs to get a new field for each parameter, that is read and written
• parameter_gui::XMLReader needs to read this field, and put it in a comment panel upon loading a file
• parameter_gui::MainWindow needs to allow modifying these comments
• parameter_gui::XMLWriter and PRMWriter need to write this field upon writing a file

[tjhei]

For your workflow dealii::ParameterHandler only needs to write this into the .xml format, right? This would avoid the problem of comments and the descriptions (that are currently printed) collide.

[gassmoeller]

Yes, this should only be written when writing xml files.

[bangerth]

Can you open corresponding issues for deal.II for the things that need to be implemented in the ParameterHandler class?

[gassmoeller]

I opened dealii/dealii#4211 to track the deal.II part.