Improve speed and readability of is_obstacle in DStarLite

[sutatoruta]

Reference issue

resolves #817

What does this implement/fix?

Building on #817, this PR improves both performance and readability by using sets for obstacle management in DStarLite.

This change reduces the computational complexity of is_obstacle from O(n) to O(1) average with respect to the number of obstacles. On my local environment, test execution time was improved from 1.72 seconds to 0.82 seconds.
While the suggested improvement in the original issue used dictionaries, I opted for sets to achieve both greater efficiency and more concise code.

Additional information

CheckList

• Did you add an unittest for your new example or defect fix?
• Did you add documents for your new example?
• All CIs are green? (You can check it after submitting)