Utility functions for working on lists of agents

[JensKanstrupLarsen]

• contains_agent_type(agents, agent_type): Returns a boolean indicating whether an agent of agent_type is in agents.
• remove_agents_type(agents, agent_type): Returns agents where all agents of type agent_type are removed.
• only_agents_type(agents, agent_type): Returns agents where all agents not of type agent_type are removed.
• agents_ordered(agents, variable, increasing=True): Returns the agents list, sorted by variable in either increasing or decreasing order. Prints an error if not all agents in the list have the attribute.
• destroy_agents(agents): Destroys all agents in agents.

[JensKanstrupLarsen]

I should mention that agents_ordered here replaces Model.agents_ordered, so a rework of some examples may be necessary.

[dybber]

Rebased on top of master