Make Solar System Edge Require Railguns

[calcwizard]

What?

Change Prometheum Science's prereq from fusion reactor to railgun.

Why?

Currently, reaching the solar system edge requires the fusion reactor research but not the railgun research. The edge can easily be reached with fission power or even with solar and a sufficient number of accumulators. On the other hand, the only practical way to reach the edge is with railguns, as the research needed to destroy huge asteroids with anything else is prohibitively expensive for that part of the game. As such, that technology requires researching superfluous technologies and allows skipping required technologies.

[crimsonarmy]

+1

Although maybe add rather than replace as new players probably should be pushed to look at fusion.