Add test coverage for libpysal/cg/kdtree.py module

[satyanvm]

libpysal/cg/kdtree.py module has 67.86% test coverage with 57/84 lines covered.

The approach to cover this test coverage I think would be to create a new test file libpysal/cg/tests/test_kdtree.py and target > 90% coverage.