Deavid: Patch 1. Various improvements.
** Game.py **
- g=Game_data now is global.
- Added definitions for work unit size, node max tech level & pipes
- some work to know when the work units were working.
** map_items.py **
- Added AutoUpgradeWhenIdle Feature for nodes and pipes.
- Added things for *_MAX_TECH_LEVEL constants
** network.py **
- Changed the way that the city is created. Now it has two nodes.
** primitives.py **
- Some constants tuned
** sound.py **
- Limited FX to play once per second as a maximum
** steam_model.py **
- Better simulation of steam model. Better precision, and less CPU usage.