make pytest and coverage work together (hacky)

- add tests/__init__.py to coax pytest-cov to report coverage

    "CoverageWarning: No data was collected. (no-data-collected)"

  is supposedly fixed with this empty
  __init__.py (see https://stackoverflow.com/a/60579142)
- inplace-installation so that coverage and pytest work with generated authors.py
  (with standard installation AND the tests/__init__.py, pytest picks up the local sources
  that does not contain the generated authors.py and fails)

Author: orbeckst

.github/workflows/gh-ci.yml

@@ -61,11 +61,11 @@ jobs:
 
     - name: Install package
       run: |
-        python -m pip install .
+        python -m pip install -e .
 
     - name: Run tests
       run: |
-        pytest -v --cov=MDAnalysisData --cov-report=xml --color=yes MDAnalysisData 
+        pytest -v --cov=MDAnalysisData --cov-report=xml --color=yes MDAnalysisData/tests
 
     - name: Codecov
       uses: codecov/codecov-action@v2

MDAnalysisData/tests/__init__.py

@@ -0,0 +1,3 @@
+# tests/__init__.py so that pytest-cov collects data
+# otherwise "CoverageWarning: No data was collected. (no-data-collected)"
+# see https://stackoverflow.com/a/60579142