These properties are necessary only if the relevant globals
are set a certain way (see-all is false, etc).
SideProperty: terrain-view layer-data...
This property is the side's current knowledge of the world's terrain.
Defaults to ().
SideProperty: unit-view layer-data...
This property is the side's current knowledge of the world.
Defaults to ().
SideProperty: unit-view-dates layer-data...
This property is the turn number at which the unit view data
in the corresponding cell of the unit-view was set.
Defaults to ().