[Spatial_searching] Allow N command-line argument in nearest_neighbor_searching example

[LOTUS9679]

Summary of Changes

This PR modifies the nearest_neighbor_searching.cpp example to make the number of neighbors (N) configurable via command-line arguments.

• Previously: N was hardcoded to 1.
• Now: The user can run ./nearest_neighbor_searching [N].
  • Added user feedback message: "generating data and searching for the N nearest neighbor(s)".
  • Defaults to 1 if no argument is provided.
  • Added #include <cstdlib> for std::atoi.

Release Management

• Affected package(s): Spatial_searching
• Feature/Small Feature (if any): Improvement to existing example
• License and copyright ownership: I confirm that I have the rights to submit this contribution under the CGAL license.