Folder structure and naming methods

As we will potentially have lot of different operators under supg, frozen velocity, P_1 viscosities, etc, I thought of adding an operator-name and folder-name to the .toml file.

For example, now the forms shear_heating and grad_rho_by_rho_dot_u (with operator name FrozenVelocity) can be grouped under terraneo folder.

Hope this will make things clean and easy to organize!