FD Solver for the Elastic Wave Equation in 2D using Staggered Grids Scheme, as per Virieux 1986

Changes are still being made ...
Fot input, "parameters.in" in build/. The matlab script generates the receivers positions ASCII file.
Same matlab script can be used to generate shot positions.
OpenMP flag needs to be used during compliation. Otherwise the program will run in serial. For CMAKE use the Cmake file as it is.

    !####################################################################################################################
    ! FD2D: Finite difference solver for the elastic wave equation in cartesian coordinates with flat free surface.
    ! NOTE: This program has not been validated.
    ! 
    ! Language: Fortran 90, with parralel impelementation using OpenMP API
    ! 
    ! Author: Mus Benziane
    ! Source: Virieux 1986
    !         other sources [2] are mentioned as comments.
    !
    ! Input file: [Example]
    ! testing              ! prefix of model file names, suffixes: _vp _vs _rho
    ! acqui_src            ! sources positions ascii file name
    ! acqui_rcv            ! receiver positions ascii file name
    ! 1996                 ! zmax
    ! 3196                 ! xmax
    ! 500                  ! nz
    ! 800                  ! nx
    ! 4                    ! dz
    ! 4                    ! dx
    ! 0.0004               ! dt
    ! 4000                 ! nt
    ! 6.                   ! Wavelet's peak frequency
    ! 50                   ! Snapshot interval
    ! 1                    ! [0/1] explosive, otherwise vertical
    ! 1                    ! [0/1] Free Surface - if set to 0, rigid BC are used, i.e no displacement at boundary
    ! 1                    ! [0/1] Absorbing
    ! .65                  ! Att constant for sponge layer
    ! 70                   ! Sponge layer width
    !
    ! -> Acquisition file in ASCII Format: 
    !  1) receivers ASCII file [N: Receivers number]
    !
    !  z1 x1
    !  z2 x2
    !  .  .
    !  zN xN
    !
    !  2) Shot points ASCII file [M: Shot points number]
    !
    !  z1 x1
    !  z2 x2
    !  .  .
    !  zM xM
    ! 
    ! -> Model files in C-Style binary floats [doubles]: Vp, Vs, Rho files are needed.
    !                                                  : For simple models, use create2Dmodel_files.f90
    ! 
    ! -> Outputs are created in OUTPUT/ if OUTPUT/ is not created by the user, the program will not handle it.
    ! Output files in OUTPUT directory:
    !
    !####################################################################################################################