Better population migration

Minor faction populations should gradually spread out from their homeworlds across empires over time, in a more organic way than they do currently. To achieve this, here a range of improvements to migration mechanics:

• Migration distances should increase. Currently populations only seem to migrate to systems that immediately neighbour their own. That range should be increased, though the rate of migration should fall off the further the distance. 

• Migration should also increase along trade routes, so that all of the systems along the route become more diverse.

• Where there is a choice, populations should tend to migrate to worlds which particularly suit them (i.e. give the greatest bonuses) if they are in range. I.e. if the Hissho homeworld is adjacent to a system with a colonised Cold planet, but there is a colonised Dry planet a bit further away, the Hissho should prefer to go further for a more appropriate climate.

• There should also be options on systems to boost or limit inward migration. There should be another option to place a system in quarantine, with no inward or outward migration at all, but with a significant hit to FIDSI production and trade.