[feature] Compute Clear Sky Index and Cloudless Index and derive Sky type

Clear Sky Index (Kc) and Cloudless Index (Cle) are two meteorological indices from Igawa (2014). They are computed from weather data in the Light.data dataframe and the CIE sky type is derived from their values, based on Figure 11 of the paper (and I chose a standard sky type for each class : overcast, intermediate overcast, intermediate, intermediate clear and clear).