HDF5 String support

[jzumer]

The HDF5 functions don't seem to currently support string datatypes.

• The +H5T-STRING class is not being checked in hdf-type->typespec in hdf-typespec, preventing loading anything with a string field at all.
• I'm having trouble locating useful documentation about how to extract the contents of a string field with the C API. It seems to work differently depending on if it's static or not and it doesn't seem to truly map to a C string even if not using the other encoding options, but that's the best I managed to find out. I can try to look at this if this is useful and if someone has an idea of where to look, though.

[ghollisjr]

I remember postponing handling HDF strings back when I made the hdf-typespec library. The data I was using encoded strings as arrays of characters, so I didn't have the direct need to handle HDF strings.

I'll look into adding support for HDF strings directly.