display result description instead of path in short output
some minor improvements
Suggestions received during the tutorial session

* For samples and results, the path is resolved/expanded before we store
it in the database. It makes more sense than relative paths.
* For results, the path may also be a file, not only a directory
* When entering a sample with no explicit #processed events, the code
tries to guess it from the parent sample and/or dataset.
Modification of the MadWeight bookkeeping

Event weights are now tracked in association with a LHCO file.
These files must be stored as (derived) samples and be directly or
indirectly associated with a dataset.

Other small improvements are also included, mostly in the behavior of
the CLI scripts.