Since the number of clear or all sky parametrizations seem to multiply, two generic generators have been implemented: clearsky and allsky that take as argument which parametrization to use. Therefore the old Brutsaert or Dilley have been removed. The Unsworth should follow at some point, after we will have added the cloud fraction as a meteo parameter and found a way to also specifiy a parametrization for clear_sky_index <-> cloudiness that could be used by the all sky generators.

Moving code around in Atmosphere (to keep the clear sky parametrizations together).