Create trace object in run object

Currently, a trace object is only created when requesting it via openml.run.get_trace(). However, it should be created at the creation time of the run object. Also, the trace object should take care of creating and reading the trace arff (as the TODO added in #481 suggests).

Part 2 of this issue is implemented by #482. Part 1 might be prohibitively expensive -> not doing this for now until there is a clear use case for this.