Viscosity alpha specification

As discussed in #524

In this boundary condition @morby we would like to change the vector alpha (viscosity.value in VT_ALPHA) with time with something like:

\alpha(Rmed[i]) = (\alpha_M-\alpha_m)*\exp(-t/T)+\alpha_m

this is not possible at the moment since alpha is defined as const

reals const& alpha = disk.physic().viscosity.alpha();

I think that the way to do is to introduce another viscosity type, and provide \alpha_M and \alpha_m as parameters. The reason is that we will also have to change the vector alpha as a function of the evolution with time of the disk temperature and density. Therefore, we will need also to change the call to the viscosity field.

But may be there is a better way to do.

@alainm : what do you suggest ?