implement compute attenuation

[lsawade]

Task description

specfem::medium::compute_attenuation

Given point::field_derivatives (member method -> strain), point::properties, point::attenuation, and attenuation_dispatch

• attenuation = none -> no op
• attenuation = RK -> compute Runge-Kutta integrated attenuation
• [Future: attenuation = RK]

Implement

R(:) = alpha(:) * R(:) + coefficient(:) * (beta(:) * strain_n + gamma(:) * strain_np1)

for each R memory variable component) slightly different between kappa and mu.

Acceptance criteria

• Small testable impl::compute_attenuation