Add calculated impedance value for each tile

[mjhouse]

Each tile should calculate impedance for units passing through:

• high water content increases impedance
• max water content while surrounded by max water tiles sets impedance=100
• ~positive steep elevation change from the previous tile increases impedance~
• ~elevation change greater than 60deg sets impedance=100~
• rocks increase impedance
• max rockiness of the tile sets impedance >= 50

[mjhouse]

Added static impedance (water and rocks) but not dynamic (direction of unit travel) in commit fb61a4bbc734df91ea929b64631e2fe914c1817f

[mjhouse]

Closing this issue as completed and creating a new issue for dynamic impedance (#19)