MAINT: Moving linux CI to a separate workflow

.github/workflows/linux_unit_test.yml

@@ -0,0 +1,54 @@
+name: Unit tests
+
+on:
+  push:
+    branches: ['**']
+  pull_request:
+    branches: ['**']
+
+jobs:
+  build:
+
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-latest]
+        python-version: [3.8,3.9]
+
+    steps:
+      - uses: actions/checkout@v4
+      - name: Set up Python ${{ matrix.python-version }}
+        uses: actions/setup-python@v5
+        with:    
+          python-version: ${{ matrix.python-version }}
+      - name: Install ubuntu dependencies
+        shell: bash -el {0}
+        run: |
+            sudo add-apt-repository ppa:ubuntu-toolchain-r/test
+            sudo apt-get update
+            sudo apt-get install libopenmpi-dev openmpi-bin
+
+      - name: Install dependencies
+        shell: bash -el {0}
+        run: |
+           python -m pip install --upgrade pip
+           python -m pip install mpi4py openmpi
+      - name: Install HNN-core
+        shell: bash -el {0}
+        run: |
+          python -m pip install --verbose '.[opt, parallel, test, gui]'
+
+      - name: Lint with flake8
+        shell: bash -el {0}
+        run: |
+          flake8 --count hnn_core
+
+      - name: Test with pytest
+        shell: bash -el {0}
+        run: |
+          python -m pytest ./hnn_core/tests/  --cov=hnn_core --cov-report=xml
+
+      - name:  Upload coverage to Codecov
+        shell: bash -el {0}
+        run: |
+          bash <(curl -s https://codecov.io/bash) -f ./coverage.xml