Performing rearrangement of Sourbron's DCDI equation, moving to new
optimization variables:


1. ETR = Ve/(Fa+Fv),
2. F = (Fa+Fv) ,
3.  fa = Fa/(Fa+Fv),
4. H = Ki/(Fa+Fv)

This might have definite advantages, due to simplified parameter ranges.
ETR is closely related to EMTT (extracellular mean transit time) and is in [0,100] sec interval.
F  is the total Inflow.

fa and H are just coefficients and should be in [0,1] range.

This seems to be a definite improvement over the {Ve,Fa,Fv,Ki }  variable set where all 4 nontrivial
intervals are required.