Replace WATER with HYDROGEN as the thruster fuel of choice.

Reduce confusion amongst players, HYDROGEN can be scooped - WATER can't, allows player to carry less of a mix of things.

We have discussed it before on the [forum](http://pioneerspacesim.net/forum/viewtopic.php?f=3&t=159#p1892) and it's recently come up [again ](http://spacesimcentral.com/ssc/topic/4310-the-front-view/) so I've quickly made and tested this change.

Finally gas giant scooping will make a bit more sense once more.