issues: 271131362

If a coordinate is passed as a set, xr raises with a bad error message:

```python

In [12]: xr.Dataset(dict(date=[1,2,3], sec={4}))

MissingDimensionsError Traceback (most recent call last) <ipython-input-12-40ccdd94e21f> in <module>() ----> 1 xr.Dataset(dict(date=[1,2,3], sec={4}))

/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/xarray/core/dataset.py in init(self, data_vars, coords, attrs, compat) 360 coords = {} 361 if data_vars is not None or coords is not None: --> 362 self._set_init_vars_and_dims(data_vars, coords, compat) 363 if attrs is not None: 364 self.attrs = attrs

/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/xarray/core/dataset.py in _set_init_vars_and_dims(self, data_vars, coords, compat) 378 379 variables, coord_names, dims = merge_data_and_coords( --> 380 data_vars, coords, compat=compat) 381 382 self._variables = variables

/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/xarray/core/merge.py in merge_data_and_coords(data, coords, compat, join) 363 objs = [data, coords] 364 explicit_coords = coords.keys() --> 365 return merge_core(objs, compat, join, explicit_coords=explicit_coords) 366 367

/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/xarray/core/merge.py in merge_core(objs, compat, join, priority_arg, explicit_coords, indexes) 425 coerced = coerce_pandas_values(objs) 426 aligned = deep_align(coerced, join=join, copy=False, indexes=indexes) --> 427 expanded = expand_variable_dicts(aligned) 428 429 coord_names, noncoord_names = determine_coords(coerced)

/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/xarray/core/merge.py in expand_variable_dicts(list_of_variable_dicts) 211 var_dicts.append(coords) 212 --> 213 var = as_variable(var, name=name) 214 sanitized_vars[name] = var 215

/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/xarray/core/variable.py in as_variable(obj, name) 103 'cannot set variable %r with %r-dimensional data ' 104 'without explicit dimension names. Pass a tuple of ' --> 105 '(dims, data) instead.' % (name, data.ndim)) 106 obj = Variable(name, obj, fastpath=True) 107 else:

MissingDimensionsError: cannot set variable 'sec' with 0-dimensional data without explicit dimension names. Pass a tuple of (dims, data) instead. But OK if a list:python In [13]: xr.Dataset(dict(date=[1,2,3], sec=[4])) Out[13]: <xarray.Dataset> Dimensions: (date: 3, sec: 1) Coordinates: * date (date) int64 1 2 3 * sec (sec) int64 4 Data variables: empty

```

Problem description

There may be reasons to not allow sets: they're not ordered, so unless you're constructing your data using the coords, you'll get random results

The error message should be better though. And I would vote to handle sets the same as lists

Expected Output

Output of xr.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 3.6.1.final.0 python-bits: 64 OS: Darwin OS-release: 17.0.0 machine: x86_64 processor: i386 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: en_US.UTF-8 xarray: 0.10.0rc1-2-gf83361c pandas: 0.21.0 numpy: 1.13.3 scipy: 0.19.1 netCDF4: None h5netcdf: None Nio: None bottleneck: 1.2.1 cyordereddict: None dask: None matplotlib: 2.0.2 cartopy: None seaborn: 0.8.1 setuptools: 36.5.0 pip: 9.0.1 conda: None pytest: 3.2.3 IPython: 6.2.1 sphinx: None