Silence warning in bundled fmt: https://github.com/fmtlib/fmt/issues/1267

Resolve "clang-format isn't set to c++17"

Closes #136

See merge request !76

Resolve "C++17 support"

This MR moves GekkoFS to C++17 with the following replacements:

- CMake modifications
- Replacing `boost::filesystem` and `boost::optional` with `std::filesystem` and `std::optional`, respectively
- Using nested namespaces
- Using `if constexpr`

Closes #134

See merge request !74

Resolve "Enforce code format"

In discussion with all developers, we decided on a code style, reflected by `clang-format`. The push includes the following changes:

- `clang-format` is now mandatory for development and code must be properly formatted to be able to push to master
- A check if the code is properly formatted by the given `.clang-format` style file, is now part of the CI pipeline test
- The style file was tested on version 10 and 12 with the style file adhering to version 10.
- A new script `scripts/check_format.sh` has been added which is used by CI but can also be used by the user for two tasks:
    - Check if any files need to be formatted
    - Pass `-r` to reformat all files in place with `clang-format`
- All code has been reformatted to the decided style 


Closes #121

See merge request !66