Refactor the `SampleModel.add()` method: split into explicit methods to either add a `SampleModel` object or create a default one.

[AndrewSazonov]

Ensure that the issue raised in this discussion is also resolved: #28 (comment)

[easyscience]

Added default label [scope] ⚠️ label needed. Please consider re-labeling this issue appropriately.

[easyscience]

Added default label [priority] ⚠️ label needed. Please consider re-labeling this issue appropriately.