Functional analysis: Add ocean

[ChristinaSchmidt1]

get_ocean()

add vignette for ocean

[ChristinaSchmidt1]

Prior Knowledge:

• Load_ocean() function
• Refactor PKN based on input data
• ocean_statistics to calculate value for each metabolic enzyme based on metabolomics
• vignette tracing (nitrogen available at the moment, but in the future carbon tracing planned to be implemented by a student)