omic::DNA is not thread safe

[Peter]

The DNA class contains static variables, which means there is a potential race condition if multiple threads reach the init() unction at the same time.

void init(void) {
  if (!char2code_.empty())
    return;
  char2code_.resize(128,255);
  code2char_ = " ACMGRSVTWYHKDBN";

  for (unsigned short i=0; i<code2char_.size(); ++i)
    char2code_[static_cast<size_t>(code2char_[i])] = i;
}

[Peter]

(In [3502]) Avoid static variables. fixes #860.