Nitrite missing in chemical_parameters_only.csv

[clairedavies]

In the chemical_parameters_only.csv file, there is no Nitrite anymore (NO2). Only nitrate (NO3) - restore nitrite to file

[leonardolaiolo]

Fix (done as part of #52):

• [x] Add Nitrite to bgc_chemistry_data.sql and combined_bgc_data.sql