Add Dataset and data variable wrappers to apply common operations upon opening

[tomvothecoder]

Is your feature request related to a problem? If yes, Please describe

CDAT users commonly work on a single TransientVariable of a dataset, which is equivalent to a single xarray Dataset data variable of type DataArray. However, an xarray data variable does not link to other data variables. This means that the extracted data variable does not have access to any existing axis bounds (also data variables).

Describe your proposed solution

Acceptance Criteria

1. Add open_dataset function to open a dataset and generate bounds if they don't exist.
   • Extensible for additional operations.
   • Add call to generate bounds if it doesn't exist
2. Add open_variable function to open a data variable with axis bounds (passed from parent Dataset). Replicates CDAT TransientVariable.
   • Extensible for additional operations
   • Add DataArray accessor to add additional attributes to DataArray class (to store bounds info)
   • Show all attributes of accessor class (just use __dict__)
   • Take into account dataset's calendar (EDIT: not needed, but keep calendar type in encoding if available)
     • I don’t think the calendar type affects the algorithm for generating a date range. Non-CF units “months since…” or “years since…” don’t factor the number of days in a month.
     • pd.date_range() generates proleptic_gregorian datetime values

Describe alternatives solutions you've considered

• On creation of the variable (easiest for the user): tas = ds.tas
  • Not possible because this is native xarray syntax, which requires extending via subclassing (not recommended) or xarray accessors.
• On dataset open: ds = xcdat.load_dataset(filename) [this would embed the bounds in the relevant variables so that they are included in ds.tas]
  • Not possible, I found Dataset data variables don't preserve custom attributes via DataArray accessors

Additional context

• Extracts some tasks for Support for time units of "months since..." #77

[tomvothecoder]

Hey @pochedls, let me know if there are any other common operations that should be performed.

[pochedls]

@tomvothecoder - an issue that arose that inspired this ticket was that xarray does not read a pretty common time unit "months since." I'm wondering if open_dataset could be used to deal with issues like this (though perhaps not in the initial release)?

[tomvothecoder]

@pochedls Thanks, this ticket extracts the easier items from #77. I'll continue the conversation specific to that issue on #77.