[FEATURE] Create homotopy decoupler

[pierre-eliep-met]

What is your precise need ?

Improve convergence through homotopy

What solution do you see ?

Add 3 decouplers : flow, pressure and enthalpy, with _in_0 and _out_0 parameters, following this model :

P_in = P_out;
Q_in = Q_out;
Xi_in = Xi_out;
h_out = homotopy(h_in, h_out_0);

and inheriting relevant flow models

Do you see any other alternative ?

It would be good to have an outer boolean parameter called use_homotopy, so that we can choose wether we want to use homotopy or not