Add coverage to unit tests

.travis.yml

@@ -62,6 +62,6 @@ script:
       ./check_source.py;
     else
       ./run_tests.py -C $OPENMC_CONFIG -j 2;
-      pytest unit_tests/;
+      pytest --cov=../openmc unit_tests/;
     fi
   - cd ..

setup.py

@@ -66,7 +66,7 @@ kwargs = {
 
     # Optional dependencies
     'extras_require': {
-        'test': ['pytest'],
+        'test': ['pytest', 'pytest-cov'],
         'vtk': ['vtk', 'silomesh'],
     },
 }