Unit tests for vecreg

Primarily testing the new --premat / --postmat options (see fdt!23 (merged))

unit_tests/fdt/vecreg/feedsRun

@@ -58,7 +58,8 @@ def main():
 
     dyads1 = Image(dyads_std).data
     dyads2 = Image(dyads_std_with_premat).data
-    assert np.isclose(cmpVectorArrays(dyads1, dyads2), 0)
+    diff = cmpVectorArrays(dyads1, dyads2)
+    assert np.isclose(diff, 0), diff
 
 
 if __name__ == '__main__':