Add forecasting capabilities

[JeffreyCovington]

Forecasting is one of the most important applications of epidemiological models, and so we want epymorph to facilitate this workflow. There are three main goals for a forecasting module:

1. The ability to simulate multiple realizations of an epidemiological model where each realization can vary by initial conditions, parameter values, and/or random number generation.
2. The ability to feed the output of a state and parameter fitting routine (such as a particle filter) into the initialization of a forecast.
3. Unifying the API for, and providing utilities for, "multi-realization" simulations including forecasting and current/future parameter and state estimation routines.