Miscellaneous-fixes

[rayosborn]

• Adds auxiliary signals to the NXdata fields added as virtual datasets in the nxconsolidate function.
• Allows links to be added to auxiliary signals defined as a NXdata attribute.
• Updates the regular expression to validate NeXus object names.
• Fixes the natural_sort function to handle path-like objects.
• Keeps the file mode as 'rw' after using the save function.
• Fixes a bug when copying virtual datasets.
• Adds a set_date function to the NXprocess class to allow the stored date to be updated.