Added CI using Github Actions

unit-tests.yml - runs unit tests for each modules
integration-tests.yml - runs integration tests, which effectively checks the overall workflow of the code in solving PDE from top to bottom

.github/workflows/integration-tests.yml

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+name: Integration tests
+
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+
+    - name: Set up Python 3.9
+      uses: actions/setup-python@v2
+      with:
+        python-version: 3.9
+
+    - name: Install dependencies
+      run: |
+        python -m venv venv
+        . venv/bin/activate
+        pip install -r requirements.txt
+
+    - name: Run integration tests
+      run: |
+        . venv/bin/activate
+        python -m pytest -v tests/integration

.github/workflows/unit-tests.yml

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+name: Unit tests
+
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+
+    - name: Set up Python 3.9
+      uses: actions/setup-python@v2
+      with:
+        python-version: 3.9
+
+    - name: Install dependencies
+      run: |
+        python -m venv venv
+        . venv/bin/activate
+        pip install -r requirements.txt
+
+    - name: Run unit tests
+      run: |
+        . venv/bin/activate
+        python -m pytest -v tests/unit