Added suffix to image stack import

Sometimes the final list of numbers in the names of a stack of images
is not the index, e.g., ‘pznpt_0001_0000.tif’. The suffix string allows
‘_0000’ to be removed before detecting the sequence. Still needs a
little testing.
Fixed root field bug

If a field is added to the file root (in violation of the NeXus
standard), NXFile._writegroup would add the root group name to the root
when writing to the file producing a duplicate tree. Also, refined the
handling of variable length string inputs to use h5py’s check_dtype
function to confirm that the object datatype is a string.