Now the environment does not deal with history directly, it delegates it to a
specific class. The analysis also uses history instances instead of either
using the database directly or creating a proxy environment.
This should make it easier to change the implementation in the future.
In fact, the change was motivated by the large size of the csv files in previous
versions. This new implementation only stores results in deltas, and it fills
any necessary values when needed.
* Graphs are not saved by default (not backwards compatible)
* Modified newsspread examples
* More granular options to save results (exporting to CSV and GEXF are now
optional)
* Updated tutorial to include exporting options
* Removed references from environment to simulation
* Added parallelism to simulations (can be turned off with a flag or argument).
