WIP: Second-order imaginary part to the dielectric function

Functions added in the DielectricFunction class to calculate the second-order contribution to the imaginary part of the dielectric function. For this, the matrix elements of the scattering potential between two Bloch states is approximated as a constant. The result is not normalized. This might be done, for example, by calculating the second-order for silver and comparing this to the experimental value of the Drude relaxation rate.

Furthermore, the function "get_eigenmodes_2nd" allows to calculate the eigenmodes of the dielectric matrix including the second-order contribution to the imaginary part.

An example and test is "test/response/2ndorder_imag_dielFunc.py".