Allow update offsets based on timestep for extdata2g

[darianboggs]

Types of change(s)

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Trivial change (affects only documentation or cleanup)
• Refactor (no functional changes, no api changes)

Checklist

• Tested this change with a run of GEOSgcm
• Ran the Unit Tests (make tests)

Description

Currently, the YAML configuration file for ExtData2 allows an offset for updates using an ISO8601 time duration string, such at T09:00:00. A user requested the capability to specify the offset as (+/-) timestep. This PR implements that capability. If map value for the update_offset is HEARTBEAT, the offset is set to an ESMF_TimeInterval equal to 1 timestep. If the map value is -HEARTBEAT, the offset is set to an ESMF_TimeInterval equal to -1 timestep. This may be extended to allow offsets of ±n timestep, where is a natural number.

Related Issue

resolve #3064

[darianboggs]

@bena-nasa and @tclune I added two tests to check if the result from using HEARTBEAT (string) matches the result from using ISO8601 Duration (string). They match for positive and negative HEARTBEAT.

Also, I added the capacity to include a + in front of the update_offset string. With the capability to use HEARTBEAT, I forsee a user using +HEARTBEAT, and I'd rather fix it now.