Consider renaming `biomass_function` routine

[hariszaf]

Should we rename this to objective function ? This way when we are not using the biomass function as objective we will not have misunderstandings.

[hritikb]

I suppose this only involves renaming the biomass_function to objective_function and then modifying the calls to biomass_function accordingly? If so, I can make these changes.

[hariszaf]

Thanks a lot @hritikb for your support. Of course you may go ahead with this task, but please let the biomass_index as is, so people can always find its index easy. Also make sure you rename this variable through the whole repo, including tests and tutorials.

[hritikb]

I have opened a PR for this (#97). You may please take a look.