Remove all warnings in VirtualFluids CPU (Closes #6, #7)

After all this MR should remove all remaining compiler warnings from VirtualFluids CPU. These warnings were mostly:

• signed/unsigned mismatches
• unused variables and parameters
• some unnecessary semicolons

.gitlab-ci.yml

@@ -99,6 +99,37 @@ gcc_9_rebuild:
     paths:
       - $CI_PROJECT_DIR/cache
 
+
+###############################################################################
+gcc_9_cpu_warning_like_errors:
+  stage: build
+
+  image: irmb/virtualfluids-deps-ubuntu20.04
+
+  tags:
+    - gpu
+    - linux
+
+  before_script:
+    - export CCACHE_BASEDIR=$CI_PROJECT_DIR
+    - export CCACHE_DIR=$CI_PROJECT_DIR/cache
+    - ccache -s
+
+  script:
+    - mkdir -p $CI_PROJECT_DIR/build
+    - cd $CI_PROJECT_DIR/build
+    - rm -r -f ./*
+    - cmake ..
+      --preset=cpu_make_ccache
+      -DBUILD_WARNINGS_AS_ERRORS=ON
+    - make -j4
+    - ccache -s
+
+  cache:
+    key: "$CI_JOB_NAME-$CI_COMMIT_REF_SLUG"
+    paths:
+      - $CI_PROJECT_DIR/cache
+
 ###############################################################################
 msvc_16:
   stage: build