Write a function named reverse_chunks that accepts a string s and integer k as parameters and returns a new string that reverses the relative order of every k characters of s. For example, the call of reverse_chunks("MehranSahami", 3) should view the string in groups of 3 characters at a time, reversing "Meh" into "he_m", and "ran" into "nar", and so on, returning a result of "he_mnarha_sima".

If the string's length is not an exact multiple of k, the last chunk of fewer-than-k characters at the end of the string should be left in its original order. For example, if the call is reverse_chunks("MartyStepp", 4), the first chunk "Mart" becomes "tra_m" and the second chunk "y_ste" becomes "et_sy". The last two characters, "pp", are fewer than 4, so they are left as-is. So the result returned should be "tra_met_sypp".