Modernize ref_app with trailing return and auto and more

[ckormanyos]

The purpose of this issue is to track potential modernization of the ref_app using trailing return type, enhanced use of auto, constexpr and more.

Some of this work has already been done. But a bunch of modernization in the mcal and util areas still remains open.

[ckormanyos]

Partly handled in #543. Most of the ref_app is now modernized. The self-written STL will simply be left as it is. The utils are nearly complete. Some MCALs are modernized but some still need modernization.

Still TODO:

• Finish modernizing the utils.
• Finish modernizing the MCALs
• Nested namespaces