Set dependency to the forked cpymad

Use the hibtc version of cpymad rather than cern-cpymad.
Remove lhcmodels locator

The lhc models are not part of the cpymad fork and there is not yet a
separate repository for these models, since the model API is still
expected to change heavily over the course of the next few months.
Switch cpymad imports to my fork of the library

The Madx class and _rpc did not change their API yet, so the change to
notebook.widget is already complete (and trivial).

The fun part will be to change the use of model.
Use new cpymad Model API in open/detail dialogs

This will be tested and corrected when I get to adapt the HIT models to
the new API.
Remove broken 'ivar' decorator

The decorator by design needs to use the name of the factory function
rather than the name of the attribute. Using multiple ivar's with the
same factory function would therefore lead to a conflict.
Adapt to cpymad not using "LookupDicts" anymore
Merge branch 'use-hibtc-cpymad-fork'