WIP save uncertainty to GWF

This series is intended to add support for writing error and uncertainty envelopes to GWF frame files. A new function is introduced to support writing frequency arrays to frame files, and then that is used in the compute_response_uncertainty() method. But this method still needs to be improved to support writing multiple frequency arrays at once, so that the error and uncertainty envelopes can both be handled.

The interface to saving files in the compute_response_uncertainty() method is also improved and simplified.