Add more details on gem5 simulation

[powerjg]

We should add some more details about event-driven simulation and what a "tick" is.

The tick rate is a constant 1ps. gem5 is an event-driven simulation and time is measured in "ticks", which is the time "quantum" (time is not continuous in gem5). Most objects in gem5 (e.g., CPU, cache, etc.) are a "ClockedObject" (they inherit from that class) and have a specified clock domain. This object-specific clock domain is what you should be modifying.