Extend Cover Calculator to hexes follwing DMG rule

[dirusulixes]

It was mentioned on discord, but I thought to add it here too as a feature request.

The DMG also has a '6-corner' rule to calculate cover at the table on hex maps that maybe could be implemented too:

On hexes, use the same procedure as a grid, drawing lines between the corners of the hexagons. The target has half cover if up to three lines are blocked by an obstacle, and three-quarters cover if four or more lines are blocked but the attack can still reach the target.