[LONGITUDINAL]
input = {parameter = {k1, k2, f0, g0}, 
         regressor = {x1, x2} }
EQUATION:
t0  = 0
f_0 = f0
g_0 = g0
ddt_f = -k1*f + k2*g + x1
ddt_g =  k1*f - k2*g + x2
