Coding guidelines are missing

[nhjschulz]

Issue:
The project is lacking coding guidelines, only formatting rules exist through clang-format.
"Pathfinder rule" is applied, but works well only for style formatting, not regarding langugage feature usage.

Impact:

• Lack of rules leads to review feedback biased to personal preferences.
• No reasoning why a construct should be used or not
• Inconsistencies when to add/remove optional C++ syntax elements that are implicit if missing .

This impairs collaboration when the number of comitters grows.

Proposed solution:

• Create/Link to coding guideline rules and improve them over time based on code review findings.