feat: Add Taper Layers in wave solvers

[acitrain]

This PR add a first version of Taper layers for wave solvers
These layers are set to improve the efficiency of absoption of boundaries.
This is how the version version works:

• We compute a taper profile which is equal to one when you are outside the taper layers and to :
  $d_x = \mathrm{exp}\left(\displaystyle \frac{3V_{\mathrm{min}}}{2L}\mathrm{log}(R)\left(\frac{L-x}{L}\right)^2\cdot \Delta t\right)$ where $V_{\mathrm{min}}$ is the minimum P-wavespeed, L the length of the taper layer and R the reflectivitity coeff
• Then we multiply by the array at time n and n+1 by the taperCoeff

For this first version only second order solver contains the taper functionality.

In a future PR, we can imagine to make the taper profile independant of the timestep and use the frequency instead.

[acitrain]

@sframba @rrsettgast @CusiniM can you review this please ?

Thanks

[Victor-M-Gomes]

This PR brings an important addition to the SEM wavepropagation module of GEOS. With this new absorbing boundary I've checked that one can achieve significantly better modelling results.

I suggest some small modifications to avoid some problems encountered during testing.

[Victor-M-Gomes]

Great job with the taper, choosing vMin over vMax was a really good choice since it usually leads to better results. This needs to be modified in the description of the PR though:
$d_x = \displaystyle \frac{-3V_{max}}{2L}log(R)(\frac{x}{L})^2$ becomes $d_x = \exp\left( \displaystyle \frac{3V_{min}}{2L}log(R)(\frac{L-x}{L})^2 dt \right)$ (the dt also appears in the definition).

After much testing the border reflections have been successfully attenuated and modelling results are significantly improved.

For the future (in another PR maybe) I would suggest substituting $V_{min} \times dt$ by some $\lambda_{min} = V_{min}/f_{max}$, where $f_{max}$ is the maximum frequency that one wishes to consider in the modelling. Right now $f_{max} == f_{sampling}$, the maximum frequency that exists in the data. By allowing the user to input $\lambda_{min}$ or $f_{max}$, the equation would have more physical meaning and the use of $dt$ could be avoided. Another option would be to set $f_{max} \simeq f_{Ricker} \times 3$ (or a similar factor) to avoid defining new inputs. The goal of the suggestion is to avoid using dt in the definition of the taper, while also inserting additional physical meaning.

Finally, it may be important to mention that the choice of L and R are somewhat empirical. For a given R, the greater the L, the better the attenuation, but that also comes with the cost of increasing the model to fit the taper. Thus, in general a compromise between L and R need to be found. For the bounds of R, I would (based on some tests) place them in the interval [ $10^{-3}$ , 1).

[sframba]

@Victor-M-Gomes I have changed the PR description accordingly

[sframba]

Re the dt, I don't find it so absurd having a dt in the formula, since if you cut the timestep in two, you want to apply the same factor as you did before for a given time. But I guess the other parameters are also heurisitc... Thoughts?

[acitrain]

@rrsettgast @CusiniM can one of you take a look to this?
It is ok on our side we just need a code owner review to go to the merge queue and do integrated test when this PR will be at the top

Thanks !