ObjHistoryClient and ObjDatasetClient cleanup

This PR contains a complete cleanup of ```gi.histories``` methods, moving implementations and documentation from ```ObjHistoryClient``` and ```ObjDatasetClient``` classes to ```History``` and ```Dataset``` respectively. Nicely, this remove 50 lines of code! After this, only ```ObjLibraryClient``` remains to be cleaned up.

Please note that 082eb66 contains an important bugfix.