Add an other mode to load 'data'

spectrum can be stored under several 'hdf5' Groups. For now the GUI does not allow to select all of those dataset. Can be like NexPy: list all values in root_node/measurement and the user select signal, axis

Then fetch data to create a list of possible spectrum. User can select the one to process.

An other input 'monitor' should be include to add an normalization after reading: new_mu = mu / monitor.

This is a view of the potential gui: