Add biological entities to owmeta to support parameter changes for aging

[slarson]

• [ ] ventral nerve cord
• [ ] motor neurons
• [ ] muscles cells
  • [ ] muscle receptors (for amplitude of PSCs)
    • [ ] excitatory nicotinic acetylcholine receptors (nAChRs)
    • [ ] inhibitory GABA receptors
• [ ] neuromuscular junctions (NMJs) in the ventral nerve cord
• [ ] Pre-synaptic motor neurons
• [ ] neurotransmitters
• [ ] muscle receptor channels
• [ ] post-synaptic currents (PSCs)
  • [ ] amplitude and frequency of PSCs
• [ ] synaptic vesicles
  • [ ] neurotransmitter uploading to synaptic vesicles
    • [ ] quantal size
      • [ ] the amount of neurotransmitter released from synaptic vesicles
    • [ ] vesicle docking/priming
      • [ ] the size of the readily releasable pool (RRP)
    • [ ] vesicle fusion

Original issue: https://github.com/openworm/OpenWorm/issues/176