Use filesystem library to deal with paths in Cerata

Copied over from: https://github.com/abs-tudelft/fletcher/issues/140

Currently the type of paths in Cerata and Fletchgen is std::string. This makes handling them not very ergonomic (see #139), which is why I'd like to suggest we use the filesystem library for this. Since we're already using C++17 features, this should not be an issue. It may however be required for some people to update their compilers.