feat: Add attenuation for acoustic wave solvers

[sframba]

This PR adds attenuation for the acoustic isotropic and VTI wave solvers, analogously as done for the elastic solvers by #3080 . THe method is similar: memory variables are added, one per standard linear solid (SLS). In the acoustic case, one variable per SLS (plus one working variable) is needed for the isotripic case, and two variables per SLS (plus two working variables) for the anisotropic case. In total $1+N$ fields are added in the isotropic case and $2+2N$ in the anisotropic case if $N$ SLSs are activated (cf. the elastic case, which needs $3+3N$ variables for both the isotrpic and anisotropic cases). All the other implementation details are the same. Attenuation is not implemented if PML are active.