Tally trigger

This is the OpenMC code with tally precision triggers and corresponding documentation. There are some changes in the tally_header file. This Pull request fulfills issue #264.
Division operations are computationally expensive. Rather than doing the division in *both* the conditional ``if`` statements and in the update of ``s % t2``, can you simply compute the relative error once and then assign it to ``s % t2`` if need be? Perhaps something like this would be simpler (and faster):

``variance = temp_t % results(1,filter_index) % trigger_sum_sq
rel_err = variance / temp_t % results(1,filter_index) % trigger_sum
std_dev = variance ** 2

if(s % t1 < variance) then 
  s % t1 = variance
end if
if (s % t2 < rel_err) then 
  s % t2 = rel_err
end if
s % t3 = std_dev``