 handles to manipulate figures (empty if plotting is
       not requested)

   EXAMPLE
   % load example data 
   data = load('example/example_data/33029_daily.mat'); 
   Q = data.Q; 
   t = data.t;
   BFI = sig_BFI(Q,t);
   BFI_LH = sig_BFI(Q,t,'method','Lyne_Hollick','parameters',[0.925 3]);
   BFI_UKIH = sig_BFI(Q,t,'method','UKIH','parameters',[5]);

   References
   Lyne, V. and Hollick, M., 1979. Stochastic time-variable
   rainfall-runoff modelling. In Institute of Engineers Australia National
   Con