GUI: Fixed saving/loading experiment state

Resolves #122 (closed) and #126 (closed)

Refactoring independently broke both saving and loading to HDF5 in the GUI. The saving problem was caused by setting the _name of the DataSet loaded to be the same as its key in the DataMap. This caused an issue in constructing the data hierarchy in HDF5 when there are forward slashes in the group name.

The Loading issue was caused by the default Experiment constructor in the GUI having an undefined Diffractometer object, which caused several issues on refreshing the various Diffractometer data members.