Add 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 #94

[mtazzari]

Ready for review.

[mtazzari]

Waiting for #89 to be merged to avoid complicated conflicts. Once #89 is merged, resume this, fix conflicts and merge.

[mtazzari]

Closing this because there're too many conflicts. I forked off develop and opened a new, cleaner, PR #111