enable the capabilty to output at flexible time levels

[guoqing-noaa]

Introduce a new output_timelevels attribute for MPAS streams that enables variable output intervals.

With this capability, we may outoput every 15 minutes in the first hour, every hour in the first 3 days, every 3 hours for the next 4 days, and every 6 hours in the last 3 days.

We can also use this to only write out forecast files during a given period, such as: output_timelevels="6-12"
Or only output mpasout files at a limited time levels output_timelevels="0 1" to facilitate cycling data assimilation

Resolve issue #1414

[guoqing-noaa]

As mentioned in Wednesday’s meeting, I created this PR for those who want to review the code changes in more detail. From my perspective, it is ready for review. That said, whether we proceed in this direction is still an open decision, and I’m fine with either outcome.

IMHO, Line 3194 of mpas_stream_manager.F clearly separates the logic between output_interval and output_timelevels, with no conflict between them.

if (stream % timelevel_spec % is_parsed) then

Therefore, both methods can coexist without issues, and users may choose whichever is most convenient for their specific applications.

@mgduda @skamaroc @liujake @byoung-joo