Computation of the drift coefficient
bx(x, fixed, random)vector of data
drift constant in front of X (when there is one additive random effect), 0 otherwise
1 if there is one additive random effect, 2 one multiplicative random effect or c(1,2) for 2 random effects
The drift is \(b(x,\phi) = \phi_1 b_1(x) + \phi_2 b_2(x)\), the output is \(b_2\) except when random c(1,2) then the output is the vector \((b_1,b_2)^t\)