[Moco] Save MocoStudy guesses specified via `TimeSeriesTable` to XML

[nickbianco]

If an initial guess is specified via a file (e.g., MocoCasADiSolver::setGuessFile), it will get saved to the guess_file property in the MocoStudy XML file. However, if the guess is set via the API (e.g., MocoCasADiSolver::setGuess), then it will not end up in the XML.

We will probably need to add a new property guess of type TimeSeriesTable to support this.