BUG: Wrong size drag law in drag.hpp

[ThomasJannaudCAM]

Describe the issue:

The drag law size is coded wrong in drag.hpp
Right now, it is
#if HAVE_ENERGY == 1 cs = std::sqrt(eos.GetGamma(VcGas(PRS,k,j,i),VcGas(RHO,k,j,i) *VcGas(PRS,k,j,i)/VcGas(RHO,k,j,i)));
which amounts to $c_s = \sqrt{ \gamma (P,P) }$.
A parenthesis should be moved, so that it becomes:
#if HAVE_ENERGY == 1 cs = std::sqrt(eos.GetGamma(VcGas(PRS,k,j,i),VcGas(RHO,k,j,i)) *VcGas(PRS,k,j,i)/VcGas(RHO,k,j,i));
which amounts to $c_s = \sqrt{ \gamma (P,\rho)P / \rho }$.

Error message:

runtime information:

To my knowledge, this is present on all versions of the code where this function is implemented.
In practice, for simulations with a constant $\gamma $ the sound speed is taken close or equal to unity.
The size drag law then becomes another gamma drag law, with $\gamma_i = 1/ \beta_i $.
I have made a couple tests with the parenthesis at the right place, such as adding dust to the AmbipolarWind test setup and checking the dust drift against terminal velocity. I found a good agreement between the two..

[glesur]

Fixed in #353