Depletion approximation solver

[dalonsoa]

Currently, the DA solver has three limitations that reduce its applicability to very simple situations and that will be useful to tackle:

1. Radiative recombination is somewhat mixed with the Shockley injection rather than as a separate contribution.

2. Can only deal with up to three layers, which have to be P-doped, N-doped and, optionally, an intrinsic layer in between. It will be useful to allow for more intrinsic layers, allowing the simulation of MQW structures, for example, using effective properties, as it is done in James P. Connolly et al, (2000), Simulating Multiple Quantum Well Solar Cells, Proc. 28th IEEE Photovoltaic Specialists Conference, Anchorage, Alaska, USA, Sept. 2000, p. 1304-1307. DOI: 10.1109/PVSC.2000.916130

3. Cannot deal with graded junctions or heterojunctions, both cases of extreme interest in modern silicon solar cells of thin film solar cells.