Depletion approximation speedup

[iclned]

The DA method presently uses a numerical integration to allow for an arbitrary photogeneration profile. This is flexible but slow. In the case of the Beer Lambert law being used for the optical model, the equations become analytical and can be evaluated much more quickly. The tasks are:

• Add a new optics model 'BLA' into solar_celll_solver.py
• Within solve_iv in solar_cell_solver.py put in a condition to check that all sub-cells are DA, no mixed methods ! Raise an error if this occurs
• Add the analytical expressions entered as a condition into depletion_approximation.py