fix buggy SegmentChainsIterator, pass tests
rename all 'balance' term to 'volume' in WeakformProcessRunnable

  - add fields of differential order in WeakformProcessRunnable
rename classes around WFAssemblier

  - change all 'WF' to 'Weakform'
  - change WeakformAssemblier.asmBalace() to WeakformAssemblier.asmVolume()
  - change all 'B/balance' to 'V/volume' accordingly