AST-1422 Check string type
When working on AST-1422, it took me way too long to realize that writing an attribute in h5py does not work intuitively.
The following creates a variable-length string, which is not handled by the H5Parm code in soltab.cc
soltab.attrs['TITLE'] = 'amplitude'
The following does work:
soltab.attrs['TITLE'] = np.void(b'amplitude')
I could not solve this issue quickly, so rather than diving in deeper, I thought I'd raise an exception to avoid falling into this trap again.