Define a default policy for single input/target file scenario

[cappelletto]

It is possible to use the same CSV file for both target (labels) and input (latents) file. However, the dataLoader object will duplicate the entries during the join operation. We can default to force join_left when a single input file is provided (or name duplication, it is equivalent)

This might require either:

• Provide the CLI option for a single input/target file definition at invocation time
• Detect name duplication at runtime and enable the join_left option

We will always assume that the input CSV (latents) contains the relevant metadata fields we want to propagate to the exported dataframe