Changes associated with gap filling using ACCESS data:
 - re-worked calculation of humidities
 - any 2 of 3 humidities (Ah,q,RH) calculated depending
   on which one is gap filled
 - allows gap filling of Ah, Rh or q
 - humidity series calculated and merged after merging of
   gap filled series so that Ta and ps available
 - implemented MergeQCFlag to allow merging (take maximum
   value) of QC flags