Fix bug for absolute paths from parser tokens.

[Alberth289346]

While adding a new language, generating reports crashed on not finding the submitted files, even though it had parsed them successfully in the analysis phase.

After several hours digging, I found that the cause was that Jplag assumes that tokens contain relative paths while generating reports. That doesn't hold for my parser. Instead, it did:

        String dir = "/path/to/exercise1";  // Command-line path to the root of submitted files
        String tokenPath = "/path/to/exercise1/stud1/x.y"; // Path stored in tokens to submitted files[X].

        File dpath = new File(dir);
        File fpath = new File(dpath, tokenPath); // As in old line 225 of Submission.java
        System.out.printf("fpath = %s\n", fpath.getPath());

which then outputs paths like fpath = /path/to/exercise1/path/to/exercise1/stud1/x.y
That of course has no hope of ever finding the file, so I patched the Submission code to deal with absolute paths from tokens.