Add GitHub Action to run tests on pull request

Run tests on Apple macOS, MS Windows, and Ubuntu Linux hosts.

macOS-hosted tests are only run on latest, which is currently macOS 10.15.
GH will eventually update latest to macOS 11 and we can look at expanding
to two versions of macOS.

Windows-hosted tests run on Windows Server 2016 and latest.

Ubuntu-hosted tests only run on latest because g++ fails to build argparse
on ubuntu-18.04 (GCC 8) since charconv was added in commit ea2f16d2.  But,
Ubuntu-hosted tests do run with g++ and clang++.

Closes #128.

Signed-off-by: Sean Robinson <sean.robinson@scottsdalecc.edu>

.github/workflows/ci.yml

@@ -0,0 +1,74 @@
+
+name: CI
+
+on: pull_request
+
+jobs:
+
+  test:
+
+    name: ${{ matrix.toolchain }}
+    runs-on: ${{ matrix.os }}
+
+    strategy:
+
+      matrix:
+
+        toolchain:
+          - macos-latest-clang
+          - ubuntu-latest-clang
+          - ubuntu-latest-gcc
+          - windows-2016-msvc
+          - windows-latest-msvc
+
+        include:
+          - toolchain: macos-latest-clang
+            os: macos-latest
+            c_compiler: clang
+            cxx_compiler: clang++
+
+          - toolchain: ubuntu-latest-clang
+            os: ubuntu-latest
+            c_compiler: clang
+            cxx_compiler: clang++
+
+          - toolchain: ubuntu-latest-gcc
+            os: ubuntu-latest
+            c_compiler: cc
+            cxx_compiler: g++
+
+          - toolchain: windows-latest-msvc
+            os: windows-latest
+            c_compiler: msvc
+            cxx_compiler: msvc
+
+          - toolchain: windows-2016-msvc
+            os: windows-2016
+            c_compiler: msvc
+            cxx_compiler: msvc
+
+    steps:
+
+    - name: Checkout Code
+      uses: actions/checkout@v2
+
+    - name: Configure
+      working-directory: test
+      run: cmake -S . -B build
+      env:
+        CC: ${{ matrix.c_compiler }}
+        CXX: ${{ matrix.cxx_compiler }}
+
+    - name: Build for ${{ matrix.os }} with ${{ matrix.compiler }}
+      working-directory: test
+      run: cmake --build build
+
+    - name: Test
+      if: ${{ ! startsWith(matrix.os, 'windows') }}
+      working-directory: test/build
+      run: ./tests
+
+    - name: Test (Windows)
+      if: ${{ startsWith(matrix.os, 'windows') }}
+      working-directory: test/build
+      run: ./Debug/tests.exe