Implement analytical depletion approximation

[phoebe-p]

If using Beer-Lambert absorption, there is an analytical solution to the depletion approximation. This is faster than the numerical (solve_bvp) or semi-analytical (Green's function) method and should be relatively easy to implement; if calling solar_cell_solver with optics_method='BL' and using the DA junction type, should default to this analytical solver (which can include front surface reflection as usual in the BL method).