Introduce convenience method to retrieve absolute path of file on local filesystem

Since our file API supports non-local filesystem there's no way of retrieving the absolute path of *any* file easily.
Still we oftentimes work with local paths and need to pass them to external tools. In these cases we should assert that we're
only using a local filesystem, and then pass the full absolute path to the external tool.
To do that more conveniently I'm adding `CM_File::getPathOnLocalFilesystem()`.

@kris-lab @tomaszdurka please review