Add description for dtkMatrix and hierarchy.
Add description for dtkMatrix and hierarchy.

NicolasSchnitzler/dtk,rdebroiz/dtk,d-tk/dtk,NicolasSchnitzler/dtk,d-tk/dtk,d-tk/dtk,d-tk/dtk,d-tk/dtk,NicolasSchnitzler/dtk,rdebroiz/dtk,rdebroiz/dtk,rdebroiz/dtk,d-tk/dtk,NicolasSchnitzler/dtk