Added a 'binding' between fluids and the network object as pn._fluids
- pn._fluids is a list that contains all fluid objects.  The fluids add themselves to this list when they are 'created'
- I decided to place the actual fluid object in the list, rather than just the 'name'
- I added two methods to the Network class which interact with the fluids objects: fluid_listing and fluid_update
- I think starting all these method with fluid (or physics, or geometry) will keep the autocomplete cleaner