NXpinhole is an invalid class in NXinstrument

[jkotan]

I get

            NXpinhole: /scan/instrument/pinhole
                NXpinhole is an invalid class in NXinstrument

It is related to nexusformat/definitions#1431

[rayosborn]

As soon as the NIAC ratify the change, I can add the NXpinhole class to NXinstrument. My original intention for NXValidate is that it help to identify issues like these that have not been resolved. If NXpinhole is already included in an application definition, then I don't believe that it should trigger an error message if you use the -a option.