EAMxx: align time indices of streams

@OlawaleClimate and I have been experimenting with reading different streams of data out once. This usually works well and xarray can combine them. However, it turns out that EAMxx puts different times for the streams of differing averaging even if they cover the same windows. Consider our example of 6-hourly-average and 6-hourly-instant streams. If we could ensure the time stamps inside these files are identical, it will be quite helpful for us!

No file, test, or entry point is named. Start by locating EAMxx handling for stream time indices and compare the 6-hourly-average and 6-hourly-instant outputs. Done means their timestamps are identical when the streams cover the same windows.

Written by the indexing model from the issue text.