Clarify in documentation that storm track input should use USA winds

[geanders]

[geanders]

The wind model code assumes that the tracking data that you input are in knots, at 10 m above the surface level, and measured using a 1-minute averaging period.

We should make sure that this is clear in the vignettes, as well as anytime that we have a function that directly inputs the original track data (e.g., get_grid_winds). If someone uses IBTrACS (https://www.ncdc.noaa.gov/ibtracs/), they can get this for a storm in any basin---they should use the column in the data called "USA WIND", which gives maximum sustained (1 minute) surface (10m) windspeed in knots. The parts here that are unique are the averaging period (1 minute) and the height (10 m).