add nutrient data to dataframe (MR !16)

• issue: nutrient field-data from 2022, 2023 and 2024 has to be added to df
• solution: create function "read_combine_nutrient_data.R"
  • adds nutrient data from Excel files to script output (goflux_data_metadata_...)
  • this is done post-processing because the nutrient data differs every year and is not available for all sites
• tested:
  • all nutrient data from the laboratory (University of Eastern Finland) from soil and water samples, namely 2022, 2023 and 2024, are read in and combined
  • all PlantRootSimulator data from 2024 are read in and combined
• some preliminary stats and visualisation added

Closes #28 (closed)