Refactored Boltzmann example docs and scripts.

[def-mycroft]

Made substantial edits to the Boltzmann wealth distribution example. The
general train of thought wasn't changed in the .md file, but many
paragraph were restructured significantly and a few paragraphs were
added.

Also, the corresponding files in examples/ were edited as well. Since
the docs page gives two examples of the Boltzmann model (with a grid and
without), two separate .jl files where created so that either
variation can be easily executed by the user. The code referenced in
docs page is exactly the same code in the two .jl files.

[kavir1698]

This is very good. Thanks for your contribution.
There is a to-do for the agent_step! function where you want the agents to exchange money only with other nearby agents. The original function already has implemented it. Specifically, the lines below:

    available_agents = get_node_contents(agent, model)
    agent2id = rand(available_agents)
    agent.wealth -= 1
    agent2 = [i for i in model.agents if i.id == agent2id][1]
    agent2.wealth += 1
    # now move
    neighboring_nodes = node_neighbors(agent, model)
    move_agent!(agent, rand(neighboring_nodes), model)

Could you add them to the function and remove the to-do before I merge this PR?

[def-mycroft]

@kavir1698 thanks for sharing the updated agent_step! code for the second part of the Boltzmann example, I'll update this and remove the TODO (will try to do it tonight, about 8 hours from now). I don't see that code in the docs on the master branch, but no matter I'll just update it now. This will close issue #27.

[def-mycroft]

Below is what I added to the Boltzmann example.

function agent_step!(agent::AbstractAgent, model::AbstractModel)
  # If the agent's wealth is zero, then do nothing.
  if agent.wealth == 0
    return
  # Otherwise..
  else
    #...create a list of all agents on the same node and select a random agent.
    available_agents = get_node_contents(agent, model)
    random_neighbor_agent_id = rand(available_agents)
    random_neighbor_agent = [i for i in model.agents
                             if i.id == random_neighbor_agent_id][1]
    # Then decrement the current agent's wealth and increment the neighbor's wealth.
    agent.wealth -= 1
    random_neighbor_agent.wealth += 1
  end
end

[def-mycroft]

@kavir1698 should the below lines be included in the agent_step! function?

   # now move
    neighboring_nodes = node_neighbors(agent, model)
    move_agent!(agent, rand(neighboring_nodes), model)

I didn't include them in my latest commit, I just didn't see where that was explained in the Boltzmann example. I can add the "move agent" lines back in if needed, I just wasn't quite sure why the agents where being moved in agent_step! and would like to explain it in the example.

[kavir1698]

Yes, agents should actually move for the model to work. Otherwise, same agents would give and receive among themselves. See this article.

[def-mycroft]

@kavir1698 , ah, I see, that is intuitive in hindsight. I added those lines to the example and wrote a comment and paragraph in the docs page mentioning this.