Improve point location

[hannesm]

Hi Peter,

I cleaned up the PointLocation implementation - similar to my Java version:

• some test cases
• Edge implements IComparable
• EdgeComparer is no longer needed -> removed
• DoubleComparer is no longer needed -> removed
• Build is reorganized to remove and insert all edges at the given x, then take a snapshot
• EndPoint structure is no longer needed, due to the dictionary from x-coordinate to lists of to-be-inserted and to-be-removed edges

Cheers,

Hannes