Avoid writing frames to disk only to be read into memory immediately

[badisa]

• This is more of a proof of concept, but looks like we can get another 10% for vacuum simulations in HREX by not writing the windows to disk until we have evaluated them, else we write them to disk then immediately read them into memory.
• The more complete way to do this would be to be able to more precisely control if the Trajectory is backed by disk or by memory