Add support for linked populations in setup file

[jessicasyu]

Adds support for specifying linked populations in the setup file:

<population id="A">
    <population.link id="B" weight="0.8" />
    <population.link id="C" weight="0.2" />
</population>
<population id="B">
    <population.link id="A" weight="1" />
    <population.link id="B" weight="1" />
    <population.link id="C" weight="1" />
</population>
<population id="C" />

Linked populations are converted to a GrabBag object, which allows you to randomly select a population from the bag with weighted probabilities. Cell agents can use linked populations to select their daughter cell population (if links are not set, the default is the same population).

In the above example:

• cells in population A have:
  • 80% change of producing a daughter cell in population B
  • 20% chance of producing a daughter cell in population C
• cells in population B have:
  • 33% chance of producing a daughter cell in population A
  • 33% chance of producing a daughter cell in population B
  • 33% chance of producing a daughter cell in population C
• cells in population C have 100% chance of producing a daughter cell in population C

Part 3 (of 3) in refactor to enable more flexibility for switching between populations.

Target branch is currently #75 to show the differences, but would need to merge #74 and #75 first and then change the base branch to develop.

[Jannetty]

resolves #72