alter unit exposure to be more in-line with original behaviour

gets rid of the exposedUnits vector, and alters the method regarding
exposure to act as "turns since spotted" rather than "turns to remember
this unit"
massive changes to cover AI procedures

ditches copy contructor and leaky memory in favour of a line calculation
to assess potential visibility. as a result, aliens will now take cover in
more logical positions.

aliens will no longer EVER seek cover as their first or second action, unless
they haven't decided what to do instead.

units that have decided to hide for the turn will take no further action past
getting to cover.

units that are taking cover will turn 180 degrees at the end of their movement
cycle instead of pathfinding for the final turn.