Cleanup

[wawando]

• Remove unused Hierarchical constructors from textfile
• Remove unused unique_id parameter from Dense class (done in template branch)
• Add const modified to non modified variables
• Use constexpr modifier for compile time constants
• Compatibility with newer C++ compiler (gcc-11) (done in Thomas' branch)
• Move OMM declarations from extension_headers into individual header file (looks clean but need to modify separate file that may not be intuitive when adding a new function)
• Remove GPU example files
• Remove TODO.md?
• Move CONTRIBUTING.md to other place?
• Rename and refactor test files to <function_name>_test.cpp?