Create a Tick View

[zhenyanghua]

Step 1 - Simple ticks, with equal interval and style, use circle for now.

• a new geometry getter method could return a list of ticks
  • each zoom level should have its own tick unit length.

zoom	tick unit (m)	tick unit (mi)
1	200,000	200
2	100,000	100
3	50,000	50
...	...	...
n	200,000/2^(n-1)	200/2^(n-1)

• The tick group should rejoin and update the data when:
  • A node add/remove/drag, line drag events happens.
  • The unit system changes.

Step 2 - Ticks with marker

• replace circle with marker
• setup the orientation for marker

Step 3 - Add ticks label based on rules.

• Update ticks data structure to include additional properties
• Add tick labels