Pass engine to xr.open_mfdataset in OpenDapSource

[lukecampbell]

When OpenDapSource._open_dataset is called when urlpath is a list of OPeNDAP URLs, no engine is specified in the call toxr.open_mfdataset even if engine is set in the class. This prevents clients from using a specified engine and reverts to xarray's default engine selection.

This commit changes the call to xr.open_mfdataset to include the engine argument as the self.engine value.

[martindurant]

+1 Do you have an easy way to test this?

[lukecampbell]

I can't seem to get the unit tests to run. They get stuck at S3 list files, but yes I have an easy way to test that involves mocks, let me see if I can get it running.

[martindurant]

They get stuck at S3 list files

I suppose you need the moto server running

[lukecampbell]

I added a simple test that simply mocks open_mfdataset and invokes the OpenDapSource calls in unit test and asserts that open_mfdataset was called with the specified engine.

[martindurant]

phew

[lukecampbell]

:+1: thanks!