Refactor damage and heal inputs

[tjsg1992]

readInput accepts a 'modifier' parameter that allows us to pass in additional information. When dealing damage, an agent should pass in the amount of damage it does (derived from some constant), and the affected agent's readInput function should use that to subtract it from the agent's health. Same for healing.

Instead, currently:

1. Knight always takes the same damage (1?) from all sources
2. Knight just heals to maximum from health potion.
3. Enemies don't take damage, they die on hit.