Implement a RK2 time stepping scheme with phase shifting

We are working with @jreneuve on a time stepping scheme doing phase shifting. I create this issue to follow this work. @jreneuve can you please use this issue to communicate on this subject?

Andrea Maffioli sent us this file phaseshifting.pdf which is very interesting but we are still not sure how to implement that in practice.

We'd like to write the algorithm following the notation of the documentation of fluidsim, see the documentation of the method _time_step_RK2. The code is here. For a first implementation, no need to use Transonic.

@jreneuve, we said that you could write a document explaining where we are and our questions. When you have something ready, can you please post it here?