removing "beams" module

Context

The module called "beams" was created in 2016 for the FYP of Jalil Shama. Unfortunately, Jalil never tried to use git during his laborious project. Eventually, the C++ translation of the initial routines (from Cedric Gommes), which was planned in the beginning of the work, has not been done. The resulting code (in MATLAB) is not linked to another module of waves and still requires much testing. It will be published somewhere else, if needed.

Description

This pull request consists in deleting the entire beams folder.

beams/attic/f_K3d_to_2d.m

-function KK2D = f_K3d_to_2d( msh, KK )
-%F_K3D_TO_2D Summary of this function goes here
-%   Detailed explanation goes here
-
-    NV = size(msh.V,1); 
-
-    % Probleme plan (3D => 2D)
-    idx32 = [6*(0:NV-1)+1; 
-             6*(0:NV-1)+2; 
-             6*(0:NV-1)+6]; % dof #1, #2 and #6
-    idx32 = idx32(:);
-    KK2D = KK(idx32,idx32');
-end
-