title: Polars Dataframe- Remove duplicate rows based on one column
body: <p>I would like to only include unique values in my polars Dataframe, based on one column.
  In the example below I would like to create a new dataframe with only uniques based on the &quot;col_float&quot; column.</p>
  <p>Before:</p>
  <pre><code>┬───────────┬──────────┬────────────┬────────────┐
  ┆ col_float ┆ col_bool ┆ col_str    ┆ col_date   │
  ┆ ---       ┆ ---      ┆ ---        ┆ ---        │
  ┆ f64       ┆ bool     ┆ str        ┆ date       │
  ╪═══════════╪══════════╪════════════╪════════════╡
  ┆ 10.0      ┆ true     ┆ 2020-01-01 ┆ 2020-01-01 │
  ┼╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌┤
  ┆ 20.0      ┆ false    ┆ 2020-01-01 ┆ 2020-01-01 │
  ┼╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌┤
  ┆ 20.0      ┆ true     ┆ 2020-01-01 ┆ 2020-01-01 │
  ┼╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌┤
  ┆ 40.0      ┆ false    ┆ 2020-01-01 ┆ 2020-01-01 │
  ┴───────────┴──────────┴────────────┴────────────┘
  </code></pre>
  <p>after:</p>
  <pre><code>┬───────────┬──────────┬────────────┬────────────┐
  ┆ col_float ┆ col_bool ┆ col_str    ┆ col_date   │
  ┆ ---       ┆ ---      ┆ ---        ┆ ---        │
  ┆ f64       ┆ bool     ┆ str        ┆ date       │
  ╪═══════════╪══════════╪════════════╪════════════╡
  ┆ 10.0      ┆ true     ┆ 2020-01-01 ┆ 2020-01-01 │
  ┼╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌┤
  ┆ 20.0      ┆ false    ┆ 2020-01-01 ┆ 2020-01-01 │
  ┼╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌┤
  ┆ 40.0      ┆ false    ┆ 2020-01-01 ┆ 2020-01-01 │
  ┴───────────┴──────────┴────────────┴────────────┘
  </code></pre>
  <p>(Notice the third row getting dropped because col_float was not unique)</p>
  <p>Intuitively, one of my attempts was:</p>
  <pre><code>let mut df = pl.DataFrame(
      {
          &quot;col_float&quot;: [10.0, 20.0, 20.0, 40.0],
          &quot;col_bool&quot;: [True, False, True, False],
          &quot;col_str&quot;: pl.repeat(&quot;2020-01-01&quot;, 4, eager=True),
      };
  let mut df2=DataFrame::new(vec![&amp;df[0]]).unwrap();
  
  df= df.unique(df2,UniqueKeepStrategy::First);
  </code></pre>
  <p>but got:</p>
  <pre><code>expected `Option&lt;&amp;[String]&gt;`, found `DataFrame`
  </code></pre>
  <p>Which was to be expected beforehand of course.</p>
  <p>I'm not sure whether im using to right function and if I do, how this subset should be passed. Searching the documentation or github did not help me as in the examples or code only &quot;None&quot; was passed as the subset.</p>