Fix: Singular Matrix error in 3D points

[akchan]

This pull request applies changes proposed by OnDraganov in the issue #6.

Problem

Singular Matrix error raises in numpy.linalg.solve() function in case of coplanar points.

Solution

Change the judge condition in the circle_contains(). OnDraganov proposed this solution in issue #6 .

Four test cases were also added to test_miniball.py.

I have confirmed the test run by the command below successfully.

# python-3.11.8
# pytest-8.1.1
# pluggy-1.4.0
pytest test_miniball.py