Add coveragepy as a dependency

CONTRIBUTING.md

@@ -0,0 +1,17 @@
+# Contributing guidelines
+
+### Running tests
+
+JANUS uses [pytest](https://docs.pytest.org/en/latest/) to run the tests. You can run the tests for yourself using:
+
+```console
+pytest
+```
+
+To check coverage:
+
+```console
+coverage run -m pytest
+coverage report  # to output to terminal
+coverage html    # to generate html report
+```

pyproject.toml

@@ -47,6 +47,7 @@ homepage = "https://github.com/FormingWorlds/JANUS"
 [project.optional-dependencies]
 develop = [
   "bump-my-version",
+  "coverage[toml]",
   "pytest"
 ]
 publishing = [
@@ -59,6 +60,10 @@ publishing = [
 package-dir = {"janus" = "src/janus"}
 include-package-data = true
 
+[tool.coverage.run]
+branch = true
+source = ["janus"]
+
 [tool.pytest.ini_options]
 testpaths = ["tests"]