Get xarray aggregation tests working about daops HOT 6 OPEN

@agstephens I've looked at these tests and found that introducing these changes doesn't cause the aggregation to break. The tests show that if the change is made on the first file in the time series then the new netcdf file includes the change otherwise the change is ignored.

https://github.com/roocs/daops/blob/issue/8/xarray_agg_tests/tests/test_xarray/test_xarray_aggregation.py

If the change is made to all 3 there is an error - I'm still looking at this and making the files temporary.

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@ellesmith88: Do the statements above relate to all the different types of changes that are tested in test_xarray_aggregation.py?

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@agstephens Yes it does, just added a second test for changing var_id to show this.

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Thanks @ellesmith88, are there any arguments that we can set to force the behaviour to be stricter?

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@agstephens Sorry I just realised this isn't the case for var_id which actually creates a new variable so that new_var_id and old_var_id are both in ds.variables no matter which file is modified.

Looking at the arguments - haven't found anything that works yet.

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@ellesmith88: that seems like a logical response, that xarray would just load two separate variables.

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