Add code coverage via Github Actions (#124)

* Add in code coverage via Github Actions and codecov.io

.coveragerc

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+[run]
+source=tobac
+omit = tobac/tests/*

.github/workflows/codecov-CI.yml

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+name: Codecov CI
+
+on: [push, pull_request]
+
+jobs:
+  run:
+    runs-on: ubuntu-latest
+    env:
+      OS: ubuntu-latest
+      PYTHON: '3.9'
+    steps:
+    - uses: actions/checkout@v2
+    # Similar to MetPy install-conda action
+    - name: Set up conda
+      uses: conda-incubator/setup-miniconda@v2
+      with:
+        miniforge-version: latest
+        miniforge-variant: mambaforge
+        channel-priority: strict
+        channels: conda-forge
+        show-channel-urls: true
+        use-only-tar-bz2: true
+
+    - name: Install dependencies and generate report
+      shell: bash -l {0}
+      run: mamba install --quiet --yes --file conda-requirements.txt coverage pytest-cov &&
+           python -m coverage run -m pytest --cov=./ --cov-report=xml
+    - name: Upload Coverage to Codecov
+      uses: codecov/codecov-action@v2
+      with: 
+        fail_ci_if_error: true
+        flags: unittests
+

tobac/tests/Dockerfile-coverage

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+FROM mambaorg/micromamba
+ARG MAMBA_DOCKERFILE_ACTIVATE=1
+
+RUN micromamba install -y -n base -c conda-forge numpy \
+    scipy scikit-image pandas pytables matplotlib iris \
+    cf-units xarray cartopy trackpy numba pytest pip \
+    pytest-cov coverage
+
+COPY . ./
+
+RUN pip install .
+
+RUN coverage run -m pytest --cov-report=xml &&\
+    mv coverage.xml shared
+