Adding new AthenaExamples package - AthExBasics

This MR adds a new package to AthenaExamples. It is intended to contain examples of tools, algorithms and config scripts for doing very basic real-world tasks such as reading the trigger decision, accessing and selecting xAOD objects, running trigger matching &c. It is hoped developers will add to these over time.

Obviously this will have no impact on any existing workflows.

This is part of the lead-up to the documentation week in April.