id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
278122300,MDU6SXNzdWUyNzgxMjIzMDA=,1749,_FillValue and missing_value not allowed for the same variable,10668114,closed,0,,,4,2017-11-30T13:37:19Z,2018-03-31T01:16:00Z,2018-03-31T01:16:00Z,NONE,,,,"#### Problem description

Currently opening the file which has a variable with both _FillValue and missing_value defined, results in a ValueError ""'Discovered conflicting _FillValue and missing_value..."", which can be overridden by setting `decode_cf=False` 

Code in `xarray/conventions.py` have comments that `missing_value is deprecated`, however this seems not to be true anymore, see [ticket #67](https://cf-trac.llnl.gov/trac/ticket/67) in http://cfconventions.org/cf-conventions/cf-conventions.html

Also, see description of meanings of _FillValue and missing_value here: 
https://github.com/Unidata/netcdf4-python/issues/94
""Note that missing_value and _FillValue are very different. _FillValue is used by the C library to fill in values in variables that have not yet been written to. missing_value is meant to represent values set by the user to represent data points that do not exist.""

#### Expected Output
Accept both _FillValue and missing_value and if both present, mask output only for _FillValue

#### Output of ``xr.show_versions()``
0.9.6

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