Change the saved structure of MDHisto from SaveMD

[rosswhitfield]

This will be version 2 of SaveMD and will change the structure of how a MDHisto will be saved. This will help users with using the output from SaveMD in other software and will make the files more NeXus compliant.

Instead of saving the 'signal' (also 'errors_squared', 'num_events' and 'mask') as a one dimensional array, it will be saved as the correct multi-dimension of array.

The axes will also be saved as a data array, not an XML string attribute of the top group.

The LoadMD will be changed to read in the new structure but will still support the SaveMD v1 format.

[rosswhitfield]

This issue was originally trac ticket 11362