CommonDataset.nc_object shouldn't try to use MFDataset on a remote dataset

[daf]

When trying to use CommonDataset.nc_object on a URL, and the first attempt at opening it fails (usually due to a 404 or similar), don't bother attempting MFDataset as it won't work remotely. Due to netcdf4-python's implementation, it won't notice until you pass a str() of the url, which is several layers down - it doesn't properly check for unicode or string.

We can error out earlier.