Low order acoustic wave equation

[mndiaye24]

First order Spectral Element Method (SEM) for the second order acoustic wave equation.

In this PR we propose the first version of the acoustic wave solver using the SEM.

The space discretization is performed using the finite element implementation provide in GEOSX. For the time integration, we use first order central finite difference scheme.

We propose an implementation with a first order Absorbing Boundary Condition (ABC).
We can also specify a free surface using the geometry boxes and field specification in the xml file.

The source injected is assumed to be a Dirac at one point inside the computational domain. It coordinate must be specified in the xml file and should be located inside one of the mesh element. It is possible to specify more than one coordinate, each coordinate will be treated as an independent source point.
Once the sources coordinate are specified, it is mandatory to specify the central time frequency for the evaluation of a Ricker function.

As for the source, the receivers locations must be specified in xml file as well.

Related to https://github.com/GEOSX/integratedTests/pull/128

sem_for_time_acoustic_equation.pdf

[mndiaye24]

Thanks @corbett5 for your review. I made some change to take into account most of your suggestion.
Now @labesse40 has merge his work on pygeosx use case in this branch. All the file he used are in the folder src/coreComponents/physicsSolvers/wavePropagation/pygeosx. Since he needed to set a current time to zero for each new shot, we have added a new condition a the beginning of explicitStep in order to update precomputed variable for the new shot.

[corbett5]

We shouldn't be committing pyc files to the repo. If its not already in the .gitignore can you add it?

[corbett5]

Do we want to be adding the .sgy files to the repo?

[rrsettgast]

Most of the suggestions I was going to provide would be better saved for a follow on PR. Just get the integrated test baseline in and merge.

[rrsettgast]

@corbett5 I think it will be necessary very shortly, so we can just leave it in.

[mndiaye24]

Do we want to be adding the .sgy files to the repo?

The .sgy are removed now.

[mndiaye24]

Hello everyone, @rrsettgast @corbett5 @francoishamon
@labesse40 has modified the python code to take into account Ben suggestion. At this stage, we decided to remove the pyhton part fron this PR. We will define a new PR only for the python and pygeox work when this first version of the acoustic solver is merged into develop. Thank you all for your review.