Only use viscosity as a field

Rigth now, the viscosity is used through its specification.

That is:

• in Disk::computeViscosity (this one is expected)
• in Disk::initRadialVelocity this one is weird,
  • the main problem here is that viscosity comes into play only starAccretion.type is not SA_WIND... how are the 2 connected ?
  • but also the value of the field is used differently depending on its type
• in DiskPhysic::Viscosity::alpha the weird thing is that both the returned value and its usage depends on the type.
• in DensityTheo

This is a sub task of #544 (closed) .