Merge branch '134-c-17-support' into 'master'

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 hpc/gekkofs!74