Morecast: Use hourly GFS values where possible

We are currently using the 1800 and 2100 hour predictions to interpolate for the local noon standard time values for the entire forecast range. GFS actually provides hourly predictions for the first 120 hours. We should use these hourly values (2000 UTC) for the first 5 days and then switch to interpolation for days 5-10.

Start by locating the Morecast forecast code that currently interpolates the 1800 and 2100 hour predictions, then trace how GFS values are selected for local noon standard time. Verify that hourly values at 2000 UTC are used through the first 120 hours and interpolation resumes for days 5-10.

Written by the indexing model from the issue text.