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 :
  $d_x = \displaystyle \frac{-3V_{max}}{2L}log(R)(\frac{x}{L})^2$ where $V_{max}$ is the maximum P-wavespeed, L the length of the taper layer and R the reflectivitity coeff (which is between $10^{-3}$ and $10^{-6}$
• 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 and the computation of the profile as been simplified in particular in corner cases.

Those two functionalities will be add in a next PR.

[acitrain]

Hi @rrsettgast

Could you take a look at it and approve if it seems good to you?
It is ready except integrated test but I want to do it after a code owner review

Thank you !

PS: I don't know why the docs build does not pass, it is only saying "We encountered a problem with a command while building your project. To resolve this error, double check your project configuration and installed dependencies are correct and have not changed recently."

[acitrain]

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

Thanks