enable processing CH4 data and add CH4 specific data-filter (MR !6)

enable processing CH4 data and add CH4 specific data-filter (MR !6 (merged))

Following filters can be (more or less) copy-pasted from N2O filter script:

• function 01: min and max length of measurement period
• function 02: remove data points (DF rows) at start and end of measurement period
  • (start and end): maximum rows of mp = 180 (chamber closure time 3 min), without absolute time
• functions 08-12 are a copy of the N2O version functions
  • 08 set unreasonable values of soil temperature to NA
  • 09 set unreasonable values of "VWC soil 12 cm" to NA (volumetric water content at 12 cm depth)
  • 10 set unreasonable values of "VWC soil 30 cm" to NA (volumetric water content at 30 cm depth)
  • 11 set PAR negative values to zero, and unreasonably high values to NA
  • 12 test for negative air temperatures in the chamber ('chamberTdegC')

NEW filters:

• function 03: difference between start and end of mp has to be at least 3 ppb, otherwise invalid mp
• function 04: if start (min) and end (max) are > 1000 ppb = bubble, flag as bubble (if possible)
  • add a new column (0 = no bubble, 1 = bubble)
  • flag in metaData DF: measPeriodMetaDataDF$detected_ch4_gas_bubble = 1
  • additional option: set mp as invalid

Testing:

• with N2O the script runs for all 2023 meas camp
• with CH4 the "Data pre-processing" runs for all 2023 meas camp

Closes #16 (closed)