Review and update warnings

[BryanRumsey]

• warning in Trajectory.__getitem__ should use log.warning and needs content updated.
• warning in Results.__getattribute__ should use log.warning and needs content updated.
• warning in Results.__getitem__ should use log.warning and needs content updated.
• warning in Results.__add__ should use log.warning and needs content updated.
• warning in Results.plot should use log.warning and needs content updated.
• warning in Results.stddev_ensemble should use log.warning and needs content updated.
• warning in Results.plot_mean_stdev should use log.warning and needs content updated.