One colonizing improvement down...

[Lincoln Peters]

I just eliminated the need for a global "ai-advanced-unit-separation"
variable by allowing the colonizer plan to decide if it's too close by
considering its distance from the other advanced unit(s).

However, I couldn't figure out how the colonizer decides what to build,
and so couldn't factor that in.  I did manage a workaround, however, by
making the desired separation equal to twice the reach of the other unit
in question.  It really should be the sum of the two unit's reaches.  So
which variable stores the unit to be built, and in which function does
it reside?  Will I have to re-prototype the good_cell_to_colonize
function so that it receives the unit to be built as an argument?


I haven't yet tested this change enough to be sure if it works properly,
but the logic is sound enough.  It looks like it works for Colonizer,
but advances and 3rd-age are more difficult to test due to their size.