This change moves the OpenDC modules previously living in the simulator/opendc
directory to the simulator directory itself given that we do not make a
distinction between OpenDC and odcsim anymore.

This change takes the first step in eliminating the explict use of
Domain and simulationContext from OpenDC. In this way, we decouple the
logic of various datacenter services from simulation logic, which should
promote re-use.

This change adds a bridge between the frontend and the new simulator
implementation via MongoDB.

This change prepares the simulator Git repository for the monorepo residing at
https://github.com/atlarge-research.com/opendc. To accomodate for this, we
move all files into a simulator subdirectory.