Return result percentage as fraction

[MasloMaslane]

Now sioworkers will return a tuple which represents a fraction of the points that the checker assigned. There are 3 formats:

• the old one: [0-9]+
• float: [0-9]+.[0-9]+
• fraction [0-9]+ [0-9]+

This will allow more precise point assignment. Related https://github.com/sio2project/oioioi/pull/332