Adds conftest.py of common fixtures for integration tests

[spjuhel]

This is based on data test used for the integration tests of the static and interpolated trajectories.

The objective is to provide:

• "as comprehensible as possible and as minimalistic as possible data (exposure, hazard, impfset)"
• Factories

Changes proposed in this PR:

• Adds fixtures and factories for minimalistic objects for integration testing
• Adds an quick example of how to use them

We need to decide if this is something we actually want.
Otherwise, I will move the content to the trajectory integration test directly.

PR Author Checklist

• Read the Contribution Guide
• Correct target branch selected (if unsure, select develop)
• Descriptive pull request title added
• Source branch up-to-date with target branch
• Documentation updated
• Tests updated
• Tests passing
• No new linter issues
• Changelog updated

PR Reviewer Checklist

• Read the Contribution Guide
• CLIMADA Reviewer Checklist passed
• Tests passing
• No new linter issues

[spjuhel]

Notes from discussion:

• shift the examples to a (possibly existing) page in the documentation