add configuration file for 'coverage'

.coveragerc

+[run]
+branch = True
+source =
+  core
+  entities
+  features
+  stadt
+  utils
+
+[report]
+exclude_lines =
+    # Have to re-enable the standard pragma
+    pragma: no cover
+    # Don't complain about missing debug-only code:
+    def __repr__
+    if self\.debug
+    # Don't complain if tests don't hit defensive assertion code:
+    raise AssertionError
+    raise NotImplementedError
+    # Don't complain if non-runnable code isn't run:
+    if __name__ == .__main__.:
+
+[html]
+directory = build/coverage-report