Describe the fundamental stock-flow concepts, variations and how our tools implement them.

  * stkflwstock - the similar process driven by demand
  * popbirths - animal population process driven by births
  * populate - animal population process driven by fertility levels
  * selfstkflw - a stock flow process whose stock depends on its’ own output
  * stkactflw - evolve a stock by additions and deletions subject to an activity
  * stkflwage - stkflwstock with entrant and exit age distributions and promotion
  * stkflwflwy - evolve a stock by year built with flows
  * stkflwio - evolve a capacity constrained input output system
  * stkflwstkshr - evolve a multityped vintaged stock to demand
  * stkflwstky - evolve a stock by year built with conversions