Many .hxx files

[flomnes]

Sometimes we have to use .hxx files to implement templates, so we have no choice.

But sometimes it's not necessary, .hxx files are not associated to templates

• src/libs/antares/study/include/antares/study/scenario-builder/rules.hxx
• src/libs/antares/study/include/antares/study/scenario-builder/sets.hxx (template StringT should be removed)
• etc.

This can lead to increased build times, since .hxx files are potentially compiled multiple times (in different CMake libraries).