Make FastSimulation faster by combining it with Simulation

[drholmie]

What is the expected enhancement

Create another simulation class, or add an option in fastsimulation that combines it with simulation to allow for speedup.
To elaborate, currently fastsimulation interprets and optionally keeps block data in a file, if instead both are run at the same time, where simulation writes into the file while fastsimluation is running, the next time it's called, it would simply read from the file (thus making it faster).
However, this doesn't necessarily hold true if one changes the code, but this enhancement can lead to future ones which can handle code changes (through the use of daemons or other clever tricks)