Add missing component and message.

Fix issue with simulation. Previously, if there was more than one tick being advanced at once, it would overshoot how many ticks it covered. So if it was covering 5 ticks and each tick happens every 100 units, rather than recording that it had simulated through t= 500, it would increase the cumulative time for each tick, recording that it had simulated through t=2500. Add error message, too.

Fix build errors.

Add issue.

Tweak money earned from donations.

Fix reference to max.

Tweak calculations. Prevent a negative number of visitors and separate visitors from donors.

Calculate entry fees and apply to budget. Incorporate simplistic model of fees.

Allow setting an entry fee. Just the structure for setting the fees.

Add very basic visitor simulation. Simulate visitors as a direct linear function of trees. This is very simplistic and looks simplistic (it's pretty easy to see visitors are always 1/10 of the trees).