Support an arbitrary number of eras and avoid hard coding the era names

[TomWerner]

This came up in discussion in https://github.com/C7-Game/Prototype/pull/644; right now we have some string->int conversions related to eras, which isn't ideal. We should figure out a better way, either by using enums, or always using ints, or something along those lines.

Genuinely supporting a generic number of eras is interesting, but we'd need to be sure to have some art name mappings in the JSON files, since the science advisor needs a mapping from era name to background pcx.