Option to place warnings into a separate error log file

[underwoo]

As the stdout of a model run can be up to several MB, it can be difficult for users to see warnings, or other important messages. Having the model output these to their own file, with some additional information (e.g., model timestep, process, etc) would be useful.

This will be in addition to the current stdlog file -- something that only contains the warnings, errors, etc.