Tech eras - allow game to start in different era

[aubergine18]

Currently, when starting a new game, we always start with the most basic tech level. It would be nice if on the game setup screen we could choose a higher tech level to skip some of the early research.

Tech levels could be something like:

* Coal
* Steam
* Electricity
* Oil
* Alien
* Space

The tech eras, and associated initial items in inventory, would be moddable - eg. we could add/remove new eras via mods.

Each era would act like a nexus in the research tree. So to progress from coal to steam, a few pre-requisite techs would be required.

If player chooses to start game at higher tech level, a few things happen:

1. All earlier techs are fully researched
2. Starting items change so they are applicable to tech level
3. Biter evolution factor is increased

This might be interesting also for MP games, to avoid very slow start players could start the game at higher tech era.