Mlforge

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- Arguments can now be "strings" that are interpreted also as literals if not found
in the host object.
- Added method (`all_argument_values`) to retrieve a list with all the values that a
given argument takes.

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- ProgressBar has been updated to refresh more accurately the progress of the entire set of nested pipelines. The API changed a little bit (different progbars are now referenced by their names instead of "ids". Whenever you call "update" you must provide the number of steps from the beginning, not the advance.

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- Now you can run multiple pipelines (with or without subtasks) one after another, updating descriptions and lenghts.