passability added to field cells
panic logging
Pathfinding, agent thinking and targeting

Added simple pathfinder (a*) and terrain. Field now can have impassable
walls and units can use pathfinder to find path to their destinations
fixed next cell check
added "sgn" util func
damsels now wander to center of cells
MoveToward to respect walls (not working still)
some debug
Slopes implemented and movement fixed

Up/down movement speed formula changed - in previous implementation
speed depended on next cell elevation and was recalculated every
movement turn. In new implementation field cell has 'slopes' attribute
which is bitfield indicating slopes on given cell. 'calcSlopeCost' used
to determine actual cost of movement through cell.

Speed calculation for movement tick now performed only once and uses
source cell modificators.
fixed NextCellCoord, reworked MoveAway
simple wall gen