Add test to validate longitude rounding and data integrity in shiftdata()

Author: Cdiaz1234

tests/test_shiftdata_rounding.py

@@ -0,0 +1,15 @@
+import numpy as np
+from mpl_toolkits.basemap import Basemap
+
+def test_shiftdata_rounding():
+    m = Basemap(projection='cyl', llcrnrlon=-180, urcrnrlon=180,
+                llcrnrlat=-90, urcrnrlat=90, lon_0=0)
+
+    lonsin = np.array([104.123456789, -75.987654321])
+    datain = np.array([1.0, 2.0])
+
+    lonsout, dataout = m.shiftdata(lonsin, datain)
+
+    expected_lons = np.round(lonsin, 6)
+    np.testing.assert_allclose(np.sort(lonsout), np.sort(expected_lons), rtol=1e-8, atol=1e-8)
+    assert np.array_equal(np.sort(dataout), np.sort(datain))