Optimize CI/CD pipeline for faster testing

What does this merge request do?

• Split the tests for py36, py37, notebooks, examples, docs and coverage into independant tasks. This way they can run in parallel and won't get blocked by each other.
• Remove the CI/CD caching. Cache is stored locally on the node on which the CI/CD job runs. These nodes are shut down once the jobs are executed. So, there is no point of caching the tox files since they are dropped anyway (this saves a few minutes of time as well).
• Remove the integration_tests job. This job is running the tests for the above mentioned six environments sequentially. Later starts a jupyter notebook (and doesn't do anything).
• Fix coverage report