Add a URL check to CI

This way we should find our earlier if a link breaks.

Author: Felix Exner

.github/helpers/check_urls.sh

@@ -0,0 +1,27 @@
+#!/bin/bash
+
+set -e
+
+#Find URLs in code:
+urls=$(grep -oP "(http|ftp|https):\/\/([a-zA-Z0-9_-]+(?:(?:\.[a-zA-Z0-9_-]+)+))([a-zA-Z0-9_.,@?^=%&:\/~+#-]*[a-zA-Z0-9_@?^=%&\/~+#-])?" "$@")
+
+fail_counter=0
+
+FAILED_LINKS=()
+for item in $urls; do
+#     echo $item
+    filename=$(echo "$item" | cut -d':' -f1)
+    url=$(echo "$item" | cut -d':' -f2-)
+    echo -n "Checking $url from file $filename"
+    if ! curl --head --silent --fail "$url" 2>&1 > /dev/null; then
+        echo -e " \033[0;31mNOT FOUND\033[32m\n"
+        FAILED_LINKS+=("$url from file $filename")
+        ((fail_counter=fail_counter+1))
+    else
+        printf " \033[32mok\033[0m\n"
+    fi
+done
+
+echo "Failed files:"
+printf '%s\n' "${FAILED_LINKS[@]}"
+exit $fail_counter

.github/workflows/check_links.yml

@@ -0,0 +1,22 @@
+name: Check Links
+on:
+  workflow_dispatch:
+  pull_request:
+  push:
+    branches:
+      - main
+
+jobs:
+  check_links:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Check URLs
+        run: |
+          .github/helpers/check_urls.sh -rI \
+            --exclude-dir=.git \
+            --exclude-dir=build/ \
+            --exclude=package.xml \
+            --exclude-dir=CMakeModules \
+            --exclude-dir=debian \
+            --exclude=ursim_docker.rst \