The field has rank 1, should be 2 for HDF5 Simple fields

[jkotan]

I get

            NXdetector: /scan/instrument/eiger9m
                Field: /scan/instrument/eiger9m/x_pixel_size
                    The field has rank 1, should be 2

when x_pixel_size is one float number written in HDF5 simple dataspace.
It should be allowed

[rayosborn]

See my comment in #14.