Add option to remove keys from registry

[tkornuta-ibm]

TL;DR: We need an option to remove keys from registry, so they won't be present in the exported config file ;)

What we are facing is a problem with reproducible testing.

• @vmarois is working on "multi-testing", that requires to add list of params [problem names etc.] into "testing" config section (PR #98 )
  • as a result, the "testing" section of the test configuration or all tests performed by our "multi-tester" will contain the same list, so running tester for one of the saved test configuration will result in running multiple tests, instead of reproducing the one.

[aasseman]

That's how I think it could be implemented properly:

• Add del_default_params and del_config_params both in ParamInterface and ParamRegistry
• In ParamRegistry, where the deletion will be actually implemented, we would need the equivalent of update_dict_recursively([...]) but for deleting, but it will be much simpler, since there will be no recursion needed. Indeed, if you remove the root of any subtree, then all of its subtrees/leaves will be removed.
• (optional, if feasible) implement __delitem__ for ParamInterface, such that you can directly do del params[key] to remove the entire subtree under key. The only issue here though is that it will be unclear (when reading the code) if it will delete the default or the config params, or both.
  • Note that this might be hard to achieve, because sometimes when doing params[key], you get a value (leaf of the tree), instead of a new ParamInterface, and I don't if it is possible to call ParamInterface's implementation of __delitem__ in that case.