externalize mdotmat
ComputeMdotMap
can be done offline, not point in doing that in the main code.
It's just a field computed from read only state of the disk.
@elena in ComputeMdotMap(GasDisk const& gas, real dt)
, dt is chosen kind of... it's the last dt os the nested loop in the simulation step (it's uniform over the whole run in our simulations).
Does it really need to be computed that often ?