fix code in neighbor_fun.jl

1. rename `neighb_fun(price::Vector, demand, clear_error, pref, budget, capacity, ind_max::Int=40)` to something that makes more sense. I don't want to see `fun` in the name of a `function`. describe what the function does!
2. Core function in that file is `decrease_demand`. This should be built with calls to `demand`. 
3. hence, this issue depends on #1 
