Clarify the introduction, discussion, and abstract to emphasize the primary goal for this study

[huddlej]

• Note that our goal is to test effects of realistic changes to public health policy and note that this study does not cover improvements to forecasting models. As part of these changes, we will include a rationale for our choice of a genetic-information-only approach rather than a model that integrates phenotypic data.
• Plot the number of total HA sequences available per region and month through the analysis period, highlighting the threshold we use for subsampling.