enable processing CO2 data and add CO2 specific data-filter (MR !10)

• issue: flux calculation & analysis does not work for CO2
• solution:
  • add option and code to preprocess, calculate and analyse CO2 data & fluxes
  • flux calculation:
    • separate R-files were added:
      • create_input_for_goflux_package_co2.R and calculate_fluxes_with_goflux_package_co2.R
    • similar to N2O/CH4 (e.g.: !9 (merged))
  • flux analysis:
    • same file as for N2O & CH4, differentiates between N2O and CH4 and CO2
• testing:
  • processing, calculation and analysis runs for all measurement campaigns for CH4, CO2, and N2O
• follow-up issue:
  • the detection and correction of "short flatlines" is not working for CO2: #27

Closes #22 (closed)