Update CPM.cmake and cache dependencies in workflows (#70)

* update CPM.cmake to 0.28.0

* cache CPM.cmake dependencies

* create modules directory if it doesn't exist on windows

* experiment with env variable

* experiment 2

* experiment 3

* use env for all workflows

* make cache keys OS dependent

* replace env.GITHUB_ with github.workspace

* format workflow files

* update cache key

* use unique key for each workflow

.github/workflows/documentation.yaml

@@ -3,7 +3,7 @@ name: Documentation
 on:
   push:
     tags:
-      - '*'
+      - "*"
 
 jobs:
   build:
@@ -12,14 +12,21 @@ jobs:
     steps:
       - uses: actions/checkout@v1
 
+      - uses: actions/cache@v2
+        with:
+          path: "**/cpm_modules"
+          key: ${{ github.action }}-cpm-modules-${{ hashFiles('**/CMakeLists.txt', '**/*.cmake') }}
+
       - name: Install dependencies
         run: |
           brew install doxygen
           pip3 install jinja2 Pygments
 
       - name: Build
+        env:
+          CPM_SOURCE_CACHE: ${{ github.workspace }}/cpm_modules
         run: |
-          cmake -Hdocumentation -Bbuild
+          cmake -Sdocumentation -Bbuild
           cmake --build build --target GenerateDocs
 
       - name: Publish