pre_process: Preprocessing of solar irradiance series Site adaptation
In cfernandezcener/SiteAdapt: Site Adaptation of Solar Irradiance Modeled Series

Preprocessing of solar irradiance series Site adaptation

pre_process(
  subset_target_period,
  latitude_target,
  z_target,
  subset_calibrating_period,
  latitude_calibrat,
  z_calibrat,
  GHI_threshold,
  DNI_threshold
)

`subset_target_period`	Dataframe object with solar radiation series to be adapted including time (with same time zone as subset_calibrating_period), the solar irradiance modeled series to be site adapted, along with their clear sky index and solar elevation (degrees)
`latitude_target`	Site latitude of solar radiation series to be adapted (degrees, +N)
`z_target`	Site elevation above sea level of solar radiation series to be adapted (m)
`subset_calibrating_period`	Dataframe object with solar radiation series for calibrating including time (with same time zone as subset_target_period), solar irradiance modeled and measured series, along with modeled clear sky index and solar elevation (degrees)
`latitude_calibrat`	Site latitude of solar radiation series for calibrating (degrees, +N)
`z_calibrat`	Site elevation above sea level of solar radiation series for calibrating (m)
`GHI_threshold`	Upper limit of GHI series (same units that Target). For automatic calulation from observed data, set it to -99
`DNI_threshold`	Upper limit of DNI series (same units that Target). For automatic calulation from observed data, set it to -99