title: question about TDengine aggregation usage
body: <p>I'm using TDengine for data processing.
  Is there someone know how to do aggregation by fixed step, not by time window?</p>
  <p>original data</p>
  <pre><code>           ts            |         val         |
  ======================================================
   2021-04-12 17:31:00.000 |              5.89000 |
   2021-04-12 17:32:00.000 |              5.86000 |
   2021-04-12 17:33:00.000 |              5.85000 |
   2021-04-12 17:34:00.000 |              5.84000 |
   2021-04-12 17:35:00.000 |              5.82000 |
   2021-04-12 17:36:00.000 |              5.84000 |
   2021-04-12 17:37:00.000 |              5.83000 |
   2021-04-12 17:38:00.000 |              5.83000 |
   2021-04-12 17:39:00.000 |              5.84000 |
   2021-04-12 17:40:00.000 |              5.84000 |
   2021-04-12 17:41:00.000 |              5.84000 |
   2021-04-12 17:42:00.000 |              5.85000 |
   2021-04-12 17:43:00.000 |              5.84000 |
   2021-04-12 17:44:00.000 |              5.85000 |
  </code></pre>
  <p>such as sum(val) by step 5，expected</p>
  <pre><code> 2021-04-12 17:35:00.000 29.26
   2021-04-12 17:40:00.000 29.18
   2021-04-12 17:44:00.000 29.18
  </code></pre>
  <p>thanks,</p>