Implement summary function for output of optimize_weights

[connormayer]

This just needs to print a slightly more user-friendly version of the output.

[adelrtan]

Would it make sense to have both the value of the objective function and the log likelihood printed in the output?
These values aren't identical once biases (or other penalty functions) are introduced.
The log likelihood may be nice to know especially if one is trying to decide between different bias values.