ExtData2G needs variant specification for update_offset

[tclune]

Currently ExtData2g supports:

update_offset: 900T

We need an option to instead specify a heartbeat offset like:

update_offset: heartbeat

(Extdata knows the heartbeat_dt.)

I'm thinking that we probably need to be able to specify multiples of heartbeat, so need a bit of exploration on best way to do this.

[tclune]

Mentioning @amdasilva so he can monitor this issue.

[darianboggs]

I'm reviewing this now.

[darianboggs]

Initially, this will use the timestep of the ExtData grid component. In the future, it may use the model heartbeat.

[tclune]

Initially, this will use the timestep of the ExtData grid component. In the future, it may use the model heartbeat.

Technically it is using the timestep of the ExtData clock. It just so happens that in MAPL2 all components share the same clock, and thus this clock ticks at the heartbeat. In MAPL3 each component has its own clock and this mechanism might become fragile if ExtData were to run less often than the heartbeat.

[bena-nasa]

Already had an issue open for this: #3000
Will close that issue