Cycle charging renewable fraction

[BenWinchester]

The cycle charging introduces the ability for the batteries within the minigrid to be charged using diesel power rather than fully from renewables. As a consequence of this, the calculation of the renewables fraction within the appraisal module no longer makes sense:

renewables_fraction = (total_renewables_used + total_storage_used) / total_energy

In order to fix this, the renewables fraction needs to take account of the fraction of battery storage which is not powered by renewables and comes from the diesel generator.