Simplify Molly interface

[tjjarvinen]

This will simplify Molly interface #34.

New interface can be used as

using Molly
using ACEmd

using ExtXYZ
using Unitful


# Load ACE model and initial structure
fname_ace = joinpath(pkgdir(ACEmd), "data", "TiAl.json")
fname_xyz = joinpath(pkgdir(ACEmd), "data", "TiAl-big.xyz")

data = ExtXYZ.Atoms(read_frame(fname_xyz))
pot = load_ace_model(fname_ace)

# Pack data to Molly compatible format
# data is AtomsBase compatible structure
sys = Molly.System(data, pot)

# Set up temperature and velocities
temp = 298.0u"K"
vel = random_velocities(sys, temp)

# Add initial velocities and loggers
sys = Molly.System(
    sys;
    velocities = vel,
    loggers=(temp=TemperatureLogger(100),)
)

# Set up simulator
simulator = VelocityVerlet(
    dt=1.0u"fs",
    coupling=AndersenThermostat(temp, 1.0u"ps"),
)


# Perform MD
simulate!(sys, simulator, 1000)

@cortner how do you like this way of building system or would you prefer something else?