Draft: average environmental variables per measurement period (MR !3)

average environmental variables per measurement period (MR !3 (closed))

• aim: one value per flux estimate for the variables -> saved into the metaDataDF
  • soil temperature (Tsoil)
  • soil moisture (VWCsoil)
• details
  • 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).

Closes #10 (closed)