Introducing Contributing guidelines and Code Quality checks

[mtazzari]

Following what we have done in the OasisLMF/OasisLMF repo, this PR:

• applies PEP8 to all the codebase;
• fixes some flake8 errors throughout the codebase (see the code-quality.yml file for more details)
• adds OasisLMF Contributing guidelines;
• adds a Pull Request template message reminding the user opening the PR to check the contributing guidelines;
• introduces automatic Code Quality workflow that checks that the Python code format is PEP8 compliant that some flake8 errors are fixed.

This PR Fixes #714

Introducing Contributing guidelines and Code Quality checks

In this PR we apply the OasisLMF-wide guidelines for code contributions and quality assurance. In particular, this PR:

• applies PEP8 to all the codebase;
• fixes some flake8 errors throughout the codebase (see the code-quality.yml file for more details)
• adds OasisLMF Contributing guidelines;
• updates the Pull Request template message reminding the user opening the PR to check the contributing guidelines;
• introduces automatic Code Quality workflow that checks that the Python code format is PEP8 compliant that some flake8 errors are fixed.

[mtazzari]

In my understanding we can merge this even if CI on jenkins fail because the new GH action workflow works and @sambles is moving jenkings CI tests on GH actions in PR #711 which is at an advanced stage.