[WIP] acoustic - elastic coupling (SEM, 2ⁿᵈ order wave equation)

[tbeltzun]

This PR is a WIP for the acoustic - elastic coupling in SEM for the second order wave equation.

PRs required:

• Add variable density inside second order acoustic wave solver #2548: required for the coupling terms;
• support higher order ricker wavelets - simplify pulse computation #2642: high order ricker wavelets;
• simplify computeSeismoTrace - add writeSeismoTrace #2641: --output cli flag fix;
• IO: filter mesh levels for VTK #2723;
• support high order hex Q2 output in VTK #2722;
• add NDEBUG test - add build_test target - fix asserts #2721;
• prevent SIGFPE in vtk #2718;
• rewrite seismo traces - rework WaveSolverBase structure #2729;
• rewrite wave propagation damping kernels #2730.

Linked issues:

• High order FEM visualization #2592: high order solution visualization (nodes and elements);

• Allocating fields on sets #166: a workaround using node sets is currently used.

• normals signs

• test slanted interface & domain swap

• validate against gar6more3d

• investigate

VTKOutput: Warning! 1 field name specified in `fieldNames` is/are not registered anywhere.
VTKOutput: Warning! `pressure_np1` is not registered anywhere and will not be displayed.

• investigate vtk crash (duplicate of [Bug] Crash when running FieldCaseTutorial1.xml #1529 (comment))

[tbeltzun]

Validation run against gar6more3d:

Acoustic receivers:

4 infinite medium (1), free surface (2), wall boundary (3) or bilayered medium(4)
0 first layer : acoustic (0) elastodynamic (1), poroelastic (2)
4 Source derivative (Hermite functions derivative, i.e. 2 for a ricker source)
10. Central frequency of the source 
1020. Amplitude of the source (× ρᶠ ?)
0.115 Delay of the source
500. Height of the source
533.3333 Height of the line of receivers (relative to interface)
2177.2936 Abscissa of the first receiver (relative to source)
0. Ordinate of the first receiver (relative to source)
2177.2936 Abscissa of the last receiver
0. Ordinate of the last receiver
1 Numbers of receivers
0. Start time
3. Stop time
1e-3 Time step (of the seismo samples)
1000 Number of intervals for the numerical computation of the convolution
1000 Number of intervals for the numerical computation of the space integration
2.295e9 1020. λᶠ and ρᶠ
1 second layer : acoustic (0) elastodynamic (1), poroelastic (2)
9.6334225e9 9.633155e9 2500. μˢ, λˢ and ρˢ

Elastic receivers:

4 infinite medium (1), free surface (2), wall boundary (3) or bilayered medium(4)
0 first layer : acoustic (0) elastodynamic (1), poroelastic (2)
4 Source derivative (Hermite functions derivative, i.e. 2 for a ricker source)
10. Central frequency of the source 
1020. Amplitude of the source (× ρᶠ ?)
0.115 Delay of the source
500. Height of the source
-533.3333 Height of the line of receivers (relative to interface)
2177.2936 Abscissa of the first receiver (relative to source)
0. Ordinate of the first receiver (relative to source)
2177.2936 Abscissa of the last receiver
0. Ordinate of the last receiver
1 Numbers of receivers
0. Start time
3. Stop time
1e-3 Time step (of the seismo samples)
500 Number of intervals for the numerical computation of the convolution
500 Number of intervals for the numerical computation of the space integration
2.295e9 1020. λᶠ and ρᶠ
1 second layer : acoustic (0) elastodynamic (1), poroelastic (2)
9.6334225e9 9.633155e9 2500. μˢ, λˢ and ρˢ

[tbeltzun]

Superseded by #2813 (cleanup).