add tests for fluence rate and attenuation interpolation

tests/test_fluence_rate.py

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+from math import dist, exp, pi
+
+import pytest
+
+from pg_rad.isotopes import CS137
+from pg_rad.landscape import Landscape
+from pg_rad.objects import PointSource
+
+
+@pytest.fixture
+def phi_ref():
+    A = 100                             # MBq
+    b = 0.851
+    mu_mass_air = 0.0778                # cm^2/g
+    air_density = 1.243                 # kg/m^3
+    r = dist((0, 0, 0), (10, 10, 0))    # m
+
+    A *= 1E9                            # Convert to Bq
+    mu_mass_air *= 0.1                  # Convert to m^2/kg
+
+    mu_air = mu_mass_air * air_density  # [m^2/kg] x [kg/m^3] = [m^-1]
+
+    # [s^-1] x exp([m^-1] x [m]) / [m^-2] = [s^-1 m^-2]
+    phi = A * b * exp(-mu_air * r) / (4 * pi * r**2)
+    return phi
+
+
+@pytest.fixture
+def test_landscape():
+    landscape = Landscape()
+    source = PointSource(
+        pos=(0, 0, 0),
+        activity=100E9,
+        isotope=CS137)
+    landscape.add_sources(source)
+    return landscape
+
+
+def test_single_source_fluence(phi_ref, test_landscape):
+    phi = test_landscape.calculate_fluence_at((10, 10, 0))
+    assert pytest.approx(phi, rel=1E-3) == phi_ref