implement CH4 flux calculation (MR !9)

• issue: flux calculation does not work for CH4
• flux pre-processing files: "H2O_ppm" from Licor needed to be added when reading in data from data logger
• run_all_main & main_flux_calculation run separately for N2O and CH4
• separate R-files were added for
  • create_input_for_goflux_package_ch4.R and calculate_fluxes_with_goflux_package_ch4
  • similar to N2O, but some small changes needed
• flux analysis: same file as for N2O, differentiates between N2O and CH4
  • IMPORTANT: new df name of the final output files!
  • date + time + goflux_ + GHG (new!) + _data_metadata__mc_202307
• testing:
  • processing, calculatin and analysis runs for measurement campaign 202307 for both N2O and CH4

Closes #21 (closed)

preprocessing/functions/calculate_fluxes_with_goflux_package_ch4.R

+# calc fluxes with GoFlux package
+# 
+#
+# input DF: goflux_input_ch4_df
+#
+# output DF: goflux_best_ch4_flux_df
+#
+
+## use DF provided by "functions/create_input_for_goflux_package.R"
+## either read input file or use DF directly
+
+