ensure gsl is installed for tests

.github/workflows/test_cogsworth.yml

@@ -26,6 +26,7 @@ jobs:
         python-version: ${{ matrix.python-version }}
     - name: Install dependencies
       run: |
+        sudo apt-get install gsl-bin libgsl0-dev
         python -m pip install --upgrade pip
         python -m pip install -e .[test]
     - name: Grab the data you need to test observables