Use date range to restrict NWIS widths download

[lekoenig]

The function summarize_nwis_widths() has optional arguments to set the date range for pulling the width data. However, we don't use those arguments in estimate_mean_width(). The implication is that different users will get different estimates, depending on the date they built the pipeline and pulled the widths down from NWIS.

For reproducibility, we should consider using date ranges in estimate_mean_width.