Spawn ratios

Infrastructure and first stab at setting up equivalency numbers to balance out spawn rates.  basically it makes more dangerous monsters count multiple times when a group is spawned, so instead of getting a group of 50 zombies with a hulk, you'll instead just get the hulk and maybe a few zombies.  A major motivator for this is so that if monster selection is adjusted, (e.g. classic zombie mode), the numbers of monsters spawned will automatically adjust to compensate.  Addresses issue #354