Minor mods and bug fixes:
 - fixed problem with chosing subsets of data when estimating
   Reco using FFNET (si and ei weren't being used)
 - implmented merge_order for MergeSeriesUsingDict after use
   of SOLO and FFNET to estimate Reco
 - modified SOLO and FFNET GUIs to make them clearer (well, I think so)
 - implemented ability to specify period as a number of months
   when estimating Reco using SOLO and FFNET
This version used to produce the first set of cummulative ET, NEE,
Fre and GPP plots from OzFluxQC output!