surface grids for negatvie/positive and inner/outer

[beomki-yeo]

@asalzburger @niermann999

I am looking into csv_io.hpp and have a question.

I understand that it reads and adds the surface grid text file line by line. It seems zphi_grid is dummy for endcaps and rphi_grid is dummy for barrels.

What I am not certain is what the differences between negative and positive (or inner and outer) are.

       // negative / positive / inner / outer
        surfaces_r_phi_grid rphi_grid_n(r_axis, phi_axis, resource);
        surfaces_r_phi_grid rphi_grid_p(r_axis, phi_axis, resource);
        surfaces_z_phi_grid zphi_grid_i{z_axis, phi_axis, resource};
        surfaces_z_phi_grid zphi_grid_o{z_axis, phi_axis, resource};

What is the motivation for the same r_phi_grid for negative/positive or z_phi_grid for inner/outer?