Clean up Map Trace Checker

[Ivan Beschastnikh]

While a lot of comments were added to the find first/ find last occurrence methods in map trace checker a while back, the code remains very dense and hard to read. That is to say it is fertile breeding ground for bugs and frustration from future developers. (In fact, we see many bugs exist in the map trace checker as is).

Some ideas:

• Refactor first and second into p and q as in main check methods.
• Refactor main check methods to take in only start position if this improves readability.
• Find first and Find last methods contain same case structure as checkers. It may be worth abstracting these to take advantage of that inheritance.

[Issue created by carolemieux: 2014-08-27]