chore: Add codecov support to ci.yml and badge to readme (#604)

.github/workflows/ci.yml

@@ -53,3 +53,5 @@ jobs:
           git remote set-url origin https://x-access-token:${{ secrets.GITHUB_TOKEN }}@github.com/$GITHUB_REPOSITORY
           git checkout $GITHUB_HEAD_REF
           git commit -am "autoformat: isort & black" && git push || true
+      
+      - uses: codecov/codecov-action@v1

README.md

@@ -2,6 +2,7 @@
 [![Code style: black](https://img.shields.io/badge/code%20style-black-000000.svg)](https://github.com/psf/black)
 [![ci](https://github.com/Deltares/HYDROLIB-core/actions/workflows/ci.yml/badge.svg)](https://github.com/Deltares/HYDROLIB-core/actions/workflows/ci.yml)
 [![Quality Gate Status](https://sonarcloud.io/api/project_badges/measure?project=Deltares_HYDROLIB-core&metric=alert_status)](https://sonarcloud.io/dashboard?id=Deltares_HYDROLIB-core)
+[![codecov](https://img.shields.io/codecov/c/github/deltares/HYDROLIB-core.svg?style=flat-square)](https://app.codecov.io/gh/deltares/HYDROLIB-core?displayType=list)
 
 
 # HYDROLIB-core