CI: Windows on GH Actions (MSVC, Clang)

Add two new builds on GH Actions: MSVC and Clang on Windows.

.github/workflows/windows.yml

@@ -0,0 +1,45 @@
+name: windows
+
+on: [push, pull_request]
+
+jobs:
+  build_win_msvc:
+    name: MSVC w/o MPI
+    runs-on: windows-latest
+    steps:
+    - uses: actions/checkout@v2
+    - name: Build & Test
+      run: |
+        md build
+        cd build
+        powershell.exe ..\share\openPMD\download_samples.ps1
+        cmake .. `
+              -DCMAKE_BUILD_TYPE=Debug
+        cmake --build . --config Debug --parallel 2
+        ctest --output-on-failure --build-config Debug
+
+  build_win_clang:
+    name: Clang w/o MPI
+    runs-on: windows-2019
+    steps:
+    - uses: actions/checkout@v2
+    - uses: seanmiddleditch/gha-setup-ninja@master
+    - name: Build & Test
+      shell: cmd
+      run: |
+        md build
+        cd build
+        Powershell.exe -File ..\share\openPMD\download_samples.ps1
+        if errorlevel 1 exit 1
+        call "C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\vc\Auxiliary\build\vcvarsall.bat" x64
+        cmake ..                            ^
+              -G "Ninja"                    ^
+              -DCMAKE_C_COMPILER=clang-cl   ^
+              -DCMAKE_CXX_COMPILER=clang-cl ^
+              -DCMAKE_BUILD_TYPE=Release    ^
+              -DCMAKE_VERBOSE_MAKEFILE=ON
+        if errorlevel 1 exit 1
+        cmake --build . --config Release --parallel 2
+        if errorlevel 1 exit 1
+        ctest --output-on-failure --build-config Release
+        if errorlevel 1 exit 1

CMakeLists.txt

@@ -716,7 +716,8 @@ endif()
 #
 # TODO: LEGACY! Use CMake TOOLCHAINS instead!
 if(CMAKE_SOURCE_DIR STREQUAL PROJECT_SOURCE_DIR)
-    if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang")
+    # On Windows, Clang -Wall aliases -Weverything; default is /W3
+    if ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang" AND NOT WIN32)
         # list(APPEND CMAKE_CXX_FLAGS "-fsanitize=address") # address, memory, undefined
         # set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -fsanitize=address")
         # set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} -fsanitize=address")