add tests for distance between objects

tests/test_objects.py

@@ -0,0 +1,36 @@
+import numpy as np
+import pytest
+
+from pg_rad.objects import Source
+
+@pytest.fixture
+def test_sources():
+    pos_a = np.random.rand(3)
+    pos_b = np.random.rand(3)
+
+    a = Source(*tuple(pos_a), strength = None)
+    b = Source(*tuple(pos_b), strength = None)
+
+    return pos_a, pos_b, a, b
+
+def test_if_distances_equal(test_sources):
+    """_Verify whether from object A to object B is the same as B to A._"""
+
+    _, _, a, b = test_sources
+
+    assert a.distance_to(b) == b.distance_to(a)
+
+def test_distance_calculation(test_sources):
+    """_Verify whether distance between two static objects (e.g. sources) 
+    is calculated correctly._"""
+
+    pos_a, pos_b, a, b = test_sources
+
+    dx = pos_b[0] - pos_a[0]
+    dy = pos_b[1] - pos_a[1]
+    dz = pos_b[2] - pos_a[2]
+
+    assert np.isclose(
+        a.distance_to(b),
+        np.sqrt(dx**2 + dy**2 + dz**2),
+        rtol = 1e-12)