Add uncertainties and subruns to powder patterns in project files

[peterfpeterson]

As pointed out in #310, the reduced diffraction data does not have uncertainties. This would also be a good time to add the extra attributes so it is plottable NeXus data. This means that the reduced diffraction data group should have additional attributes:

• NX_class with value NXdata
• signal with value main
• axes with value ["2theta", "subrun"]

There should be additional fields

• subrun
• 2theta gets a units attribute
• main_error

[fanchercm]

data uncertainty was added in #403