Added input and mesh files for Cext calculations

    * Added mesh files for sphere with 512 elements at different locations
    * Added input file for seven sphere case
    * Added script to generate sphere meshes with different radius and locations
    * Added script to interpolate diffraction coefficient (linear interpolation)
    * Added files with diffraction coefficients for:
        * Gold, Silver from Johnson, Christy 1972
        * Water from Halle, Querry 1972
    * Added script to compute protein diffraction coefficient
        * Relation taken from Paliwal, Tomar, Gupta 2014