Tally derivation

This is the OpenMC code with tally precision triggers and corresponding documentation. This Pull request fulfills issue #264. 
@paulromano  I have changed my code to following:
type TriggerResult
     real(8) :: trigger_sum=0.
     real(8) :: trigger_sum_sq=0.
   end type TriggerResult
But when I allocate I have to allocate it as: 

allocate(temp_t % results(t % total_score_bins, t % total_filter_bins)) 
temp_t is just defined once, allocated and deallocated for each i i.e. the i of tallies(i)