data_path should be auto populated

[cmdoret]

When adding a data object into an existing MODO, the data_path property indicates the filepath inside the object. This must currently be filled by the user, but in most use cases, it will be the same filename as input. We should use the input filename as default value.

Objective: Use a default value, either from the input filename, or by combining id and extension.

Requirements:

• Check behaviour in case of filename collision
• Change the API to use the input filename if collisions are accounted for.