Save model output summary to file

[sicasley]

OOB error gets displayed on screen when running wetland_model(), but there is no lasting record of this. In addition to saving diagnostics (#11), it would be useful to save a summary of the model output - which includes OOB error - to file for reference. This can be achieved by running

> print(model.out)

To save this output to file, for example:

> sink("./output/20190703-130403/20190703-130403_model_results.txt")
> print(model.out)
> sink()

Look at incorporating this as part of wetland_model().