average environmental variables per measurement period

• Aim: one value per flux estimate for the variables -> saved into the metaDataDF
  • soil temperature (Tsoil)
  • soil moisture (VWCsoil)
• Tsoil:
  • calculate average (not median) from soil temperature sensors 1-4 from second 180 until second 600
• VWCsoil_12cm:
  • calculate average (unsure) from second 180 until second 600
• VWCsoil_30cm:
  • Calculate average (unsure) from second 180 until second 600.
  • all values could be NA (this would be the case if the soil was frozen and the sensor could not be inserted into the soil to that depth).