title: count the number of a certain triplet in a file (DNA codon analysis)
body: <p>This question is actually for DNA codon analysis, to put it in a simple way, let's say I have a file like this:<br>
  atgaaaccaaag...<br>
  and I want to count the number of 'aaa' triplet present in this file. Importantly, the triplets start from the very beginning (which means atg,aaa,cca,aag,...) So the result should be 1 instead of 2 'aaa' in this example.<br>
  Is there any Python or Shellscript methods to do this? Thanks!</p>