add option to save observed streamflow data used for calculating performance metrics

[Ben-Choat]

Short description explaining the high-level reason for the new issue.

ENHANCEMENT

Current behavior

The ngen-cal cod euses HY_Features to access nwis data to get usgs streamflow data. It uses this data to calculate performance metrics against the simulated streamflow. The data is not saved though.

Desired behavior

Include options in the calibration configuration file to 1. Specifiy if the user would like to save the observed data and 2. if 1 is True, provide a directory in which to save the data.

[aaraney]

@hellkite500, I think we can accomplish this and support other use cases if we create something like an ngen_cal_model_evaluate hook. A plugin that returns None could be used in this case just to read the values that are passed to evaluate. Its not perfect, but I think we should consider it.

[aaraney]

@Ben-Choat, once #155 is merged you will be able to add this functionality via ngen.cal's plugin system. See #155 for an example.