Make the names more understandable

Here are some (minor) suggestions that will improve the readability of the code while going through the repo.

https://github.com/TulipaEnergy/NearOptimalAlternatives.jl/blob/main/src/alternative-optimisation.jl:

• l 23-25: maybe call them initial_objective* (or original*)
• l 34: solution -> initial_solution
• l 40: quite confusing to call a constraint original_objective, so original_objective -> optimality_gap_constraint
• l 68: better to explain a bit more, e.g., by adding something like so the sum of the distance between the new solution and each of the previous solutions is maximized to the comment
• l 69: solution -> previous_solution

https://github.com/TulipaEnergy/NearOptimalAlternatives.jl/blob/main/src/results.jl:

• l 31: shouldn't it be push!(results.objective_values, objective_value(model))? What do we want to store here?

https://github.com/TulipaEnergy/NearOptimalAlternatives.jl/blob/2-mga-metaheuristics/src/generate-alternatives.jl:

• l108: more explicit name?