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--  Program structure:  --
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1. build n-body system 
    - contains grids, couplings, gridViews, assembler 
    
    Data structure: LevelContactNetwork
    Factories:      StackedBlocksFactory
    
2. initialize/set up program state
    - holds bodyStates, u, v, a, alpha for each body
    - defines time, timeStep
    - computes initial conditions
    
    Data structure: ProgramState

-- tested until here

3. assemble RSD friction

4. set up TNNMG solver
    - rate updater
    - state updater
    
5. adaptive time stepper