Add testing step: Run some local checks before going distributed

[JavierCVilla]

For a given user analysis, it would be nice to run some local checks so users can discover possible issues in their code before connecting to a cluster. Possible common errors may be:

• Non-serializable functions passed to the PyRDF.initialize method.
• Non-correct cpp functions in RDataFrame operations.
• Inaccessible input files

These errors can be easily discovered running a reduced version of the analysis for a few entries with a Local backend while running on a given backend the error message might not be that clear.

[vepadulano]

The Local backend has been removed, but this can still be implemented by calling plain RDF code