User integration of the SDE for the user-defined variables.
The variables are constant by default. The SDE must be of the form:
T: particle attribute representing the variable
Tca: characteristic time for the sde to be prescribed in the array auxl1
PIP: coefficient of the SDE (pseudo RHS) to be prescribed in the array auxl2.
If the chosen scheme is first order (nordre=1) then, at the first and only call pip is expressed as a function of the quantities of the previous time step (contained in the particle data).
If the chosen scheme is second order (nordre=2) then, at the first call (nor=1) pip is expressed as a function of the quantities of the previous time step, and at the second passage (nor=2) pip is expressed as a function of the quantities of the current time step.
Example of the integration of the SDE (Stochastic Differential Equation).