p5js/mitosis - Add capability for more than one cell
p5js/mitosis - Add capability for more than one cell

* This is preparation of splitting the cell (Mitosis!!) into two
* Minor refactor of 'this.cell.fluidity' into 'this.cellFluidity' variable within the CellMembraneSegment ... this vibration code will likely be reworked altogether.
  - and I don't want each MembraneSegment to hold on to a reference to its Cell; because that would have to be maintained ... don't see the value yet.

brianhonohan/sketchbook,brianhonohan/sketchbook,brianhonohan/sketchbook