Better disc initial conditions

[becnealon]

In the current version when a disc is run there is always a transient from the initial condition. Visually this looks like the disc takes an inhale followed by an exhale. I think this is because the initial velocities are not quite perfect, so the disc has to adjust.

This effect is made worse by using self gravity and independently with MCFOST. For MCFOST the disc thickness decreases dramatically, manifested by a wave moving from the inner edge out. Dust presents an additional problem as the initial dust thickness is not in agreement with the analytical profile.

The goal is to remove the initial transient so that we don't have to run a relaxing stage for simulations. I think we can do this using the relax procedure that is used in common envelope calculations and APR. Ideal solution is something that is generalised for whatever setup you run, e.g. in the presence of a flyby, +/- MCFOST, +/- self gravity.