choose between HDKR and isotropic sky model
model Skymodel = TransiEnt.Producer.Heat.SolarThermal.Base.SkymodelBase(longitude_local = longitude_local, longitude_standard = longitude_standard, latitude = latitude, slope = slope, surfaceAzimuthAngle = surfaceAzimuthAngle, reflectance_ground = reflectance_ground, direct_normal = direct_normal, totaldays = totaldays);