Agents can spawn in walls

[georggrab]

For some field sizes, agents can spawn in walls. The agent seems to have priority, so can move out of the wall, but once you move out of the wall, you can't move back in. This is giving the agent that happens to spawn in a corner a disadvantage, as it can basically only blow itself up.

Steps to reproduce:

• in settings.py, set COLS=8, ROWS=8.
• play with 4 agents
• not always, but sometimes, the agent in the bottom right corner will spawn in a wall. this agent is significantly disadvantaged (lower winning fraction if for example letting 4 rule_based_agents play against each other)