Add Reference Characteristic definition and shape.

[Yauhenikapl]

Description

Please include a summary of the changes and the related issue. List any dependencies that are required for this change.

Fixes #138

Type of change

• New feature (non-breaking change which adds functionality)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works