title: how to calculate max value in some columns per row in pyspark
body: <p>I have a data frame read with sqlContext.sql function in pyspark.
  This contains 4 numerics columns with information per client (this is the key id).
  I need to calculate the max value per <code>client</code> and join this value to the data frame:</p>
  <pre><code>+--------+-------+-------+-------+-------+
  |ClientId|m_ant21|m_ant22|m_ant23|m_ant24|
  +--------+-------+-------+-------+-------+
  |       0|   null|   null|   null|   null|
  |       1|   null|   null|   null|   null|
  |       2|   null|   null|   null|   null|
  |       3|   null|   null|   null|   null|
  |       4|   null|   null|   null|   null|
  |       5|   null|   null|   null|   null|
  |       6|     23|     13|     17|      8|
  |       7|   null|   null|   null|   null|
  |       8|   null|   null|   null|   null|
  |       9|   null|   null|   null|   null|
  |      10|     34|      2|      4|      0|
  |      11|      0|      0|      0|      0|
  |      12|      0|      0|      0|      0|
  |      13|      0|      0|     30|      0|
  |      14|   null|   null|   null|   null|
  |      15|   null|   null|   null|   null|
  |      16|     37|     29|     29|     29|
  |      17|      0|      0|     16|      0|
  |      18|      0|      0|      0|      0|
  |      19|   null|   null|   null|   null|
  +--------+-------+-------+-------+-------+
  </code></pre>
  <p>In this case, the max value to the client <code>six</code> is <code>23</code> and the client <code>ten</code> is <code>30</code>. the <code>null</code> is naturally null in the new column.</p>
  <p>Please help me showing how can i do this operation.</p>