title: pandas add datetime column
body: <p>I have a dataframe where year, month, day, hour is split into separate columns:</p>
  <pre><code>df.head()
      Year   Month   Day  Hour
  0   2020   1       1    0
  1   2020   1       1    1
  2   2020   1       1    2
     ...
  </code></pre>
  <p>I'd like to add a proper datetime column to the dataframe so I end up with something along these lines:</p>
  <pre><code>df.head()
      Year   Month   Day  Hour datetime
  0   2020   1       1    0    2020-01-01T00:00
  1   2020   1       1    1    2020-01-01T01:00
  2   2020   1       1    2    2020-01-01T02:00
     ...
  </code></pre>
  <p>I could add a loop that processes one row at a time, but that's not panda-esque.
  Here are three things that don't work (not that I expected any of them to do so):</p>
  <pre><code>df['datetime'] = pd.to_datetime(datetime.datetime(df['Year'], df['Month'], df['Day'], df['Hour']))
  df['datetime'] = pd.to_datetime(df['Year'], df['Month'], df['Day'], df['Hour'])
  df['datetime'] = pd.datetime(df['Year'], df['Month'], df['Day'], df['Hour'])
  </code></pre>