title: NotImplementedError is thrown when I use isin with Dask data frames
body: <p>Let's say I have two dask data frames:  </p>
  
  <pre><code>import dask.dataframe as dd 
  import pandas as pd
  
  dd_1 = dd.from_pandas(pd.DataFrame({'a': [1, 2,3], 'b': [6, 7, 8]}), npartitions=1)
  
  dd_2 = dd.from_pandas(pd.DataFrame({'a': [1, 2, 5], 'b': [3, 7, 1]}), npartitions=1)
  </code></pre>
  
  <p>Now I want to filter the first one using the values of the column in the second one:</p>
  
  <pre><code>dd_1[dd_1.a.isin(dd_2.a)]
  </code></pre>
  
  <p>When I try to do this the following error is thrown:</p>
  
  <pre><code>NotImplementedError                       Traceback (most recent call last)
  &lt;ipython-input-38-850f035e0842&gt; in &lt;module&gt;
  ----&gt; 1 dd_1[dd_1.a.isin(dd_2.a)]
  
  /usr/local/lib/python3.7/site-packages/dask/dataframe/core.py in isin(self, values)
     2113     @derived_from(pd.Series)
     2114     def isin(self, values):
  -&gt; 2115         return elemwise(M.isin, self, list(values))
     2116 
     2117     @insert_meta_param_description(pad=12)
  
  /usr/local/lib/python3.7/site-packages/dask/dataframe/core.py in __getitem__(self, key)
     2045             graph = HighLevelGraph.from_collections(name, dsk, dependencies=[self, key])
     2046             return Series(graph, name, self._meta, self.divisions)
  -&gt; 2047         raise NotImplementedError()
     2048 
     2049     @derived_from(pd.DataFrame)
  
  NotImplementedError: 
  </code></pre>
  
  <p>Any suggestion?</p>