calculate mean and std

You want to find the maximium likelihood standard deviation and mean.

It might look like this:

my ($mean, $ssq, $count) = (0, 0, 0);
foreach my $i (keys %sample_coverage)
{
  my $depth = $sample_coverage{$i} / $tot_bases;
  $mean += $depth;
  $ssq  += $depth * $depth;
  $count++;
}
$mean /= $count;
my $std = sqrt( ( ($ssq / $count) - $mean * $mean) );