Update of coding guidelines for cpp

[sandro-elsweijer]

Currently, our coding guidelines only describe function and class/struct naming conventions and we need to adapt these to all other cpp types like template parameters and member functions.

I propose the following conventions, which are also used in Boost and the STL. These would mean minimal changes to our code and the snake_case would allow our spell checker work properly. CamelCase and pascalCase often do not work with spell checkers.

• structs/classes/free functions: t8_snake_case with t8 prefix (t8_forest_..., t8_geometry_...)
• member functions and member variables: snake_case without t8 prefix (the t8 prefix is already in the class name)
• Template parameters: TCamelCase with leading T for template

Furthermore, we should think about the _t in our typedefs, since it is reserved for POSIX.

Alternatively, we could also completely remove the t8 prefixes and introduce namespaces, but this would be a bigger change.

The longer we wait, the more we have to change.

Edit:
I stmbled upon some functions, which do this: returnType t8_some_function(void); From my short research this is the way to go, but we do not do this consistently. Should we adapt it to all other functions which have no arguments?