GUI v0.3

Many new additions in this one again:

• A graph to display population against months
• Some bug fixes
• Some more code documentation in pop_sim_rust_gui/src/main.rs
• And the beginnings of a more detailed simulation with: fertility! (the fertility values will be tweaked occasionally to give space for future systems)

Note: now that the simulation has fertility you may need to run the simulation more than once as it may not always exponentially grow and instead fall to a population of zero...