Feature: Include additional options for tracking ticks/age for individual agents

[cainja]

Problem Description

Currently in patch, agent age are tracked and never reset in the simulation, even after proliferation. This age is used to determine probability of apoptosis if the age > death_age. This behavior leads to the following:

1. Agent ages are never reset and therefore a population cannot replenish itself.
2. The distribution of agent ages shifts over time as a technical artifact rather than reflecting biological processes.
3. There exists potential artifacts from the time scales of the simulation and cellular death.
4. Control over these population dynamics is possible by carefully considering the initialization of cell age and death age.

However, the documentation is somewhat confusing surrounding the function of cellular age; controlling population dynamics via initialization and death age can obscure the biological meaning of age/death age.

Proposed Solutions

1. Improve documentation and intuition describing purpose and initialization of cell age and death age.
2. Include additional parameter (similar to cycle time) that describes ticks since last division/creation in additional to currently tracked age parameter.

Additional Discussion Notes

We briefly discussed changing the behavior of cells, where

1. Daughter cells' ages is reset leading to a shift in the distribution rather than desired steady state snapshot of the cell age distribution
2. Resetting a single daughter cell's age would maintain the distribution but is unintuitive to interpret ages
3. Achieving the desired behavior here is possible by adjusting parameters and initialization therefore it seems unnecessary to make logic changes to the code but improving documentation and clarity seemed to address the issue