Add stride option to load_data()
	modified:   hdf5.py
Added gas phase plotting functions.
	modified:   snapshot.py
New module(class) sim(Simulation) for simulation metadata and organization.
	modified:   __init__.py
	new file:   sim.py
Updated snapshot.File to work with sim module.
	modified:   snapshot.py
Updated to work with sim.Simulation.
	modified:   scripts/gas_phase.py