Created scenario capabilities

New modelinit file (modelinit_scenario.R) takes, as an input, a scenario
object that contains all the inputs except government interventions.
New file that creates that scenario object.

This re-organization of the initiation code should allow for the saving
and repeating of specific scenarios.
easily changable filenames

created a filename variable, so that's easier to change
also, several commented-out lines of code allow for saving the model
inputs if desired