updates Entity model

restrictions to only query and add resources
adds Change model

class to specific changes to an Entity
adds ChangeTracker class

class to accumulate changed resource amounts
adds ChangeManager

to handle Change instances
fix bug in get methods
able to fetch entity/change from tag
adds Simulator class

basic functionality
updates to model classes

- simulator relies on retriving entities, getting each of their tags,
  then getting changes for each tag