[LinearSystem] Cache mapped mass matrix (#4625)* [LinearSystem] Cache mapped mass matrix* revert observer pattern* track mass change using DataTracker* Verify J is constant by introducing isLinear method* d_recomputeCachedMassMatrix is now EmptyData* Comments* comments* track changes with DataTracker* Try to fix Ubuntu compilation* Missing matrix compression* Also track mappings
[GUI.Qt] Add clickable link to online documentation (#4650)* [GUI.Qt] Add clickable link to online documentation* Revert adding hard-coded url* Deduce URL automatically from namespace and class name* Define URL at the toplevel