Rename dtype alignment property to data_alignment

This makes it more consistent with data_size
Add exposure of the dynd metadata struct to the python lowlevel namespace
Check the notebooks against recent changes
Rename the 'lowlevel' submodule to '_lowlevel'

to more clearly indicate that it's "internal", not
for typical dynd usage.
Add a notebook illustrating the dynd type/metadata connection

with worked examples using ctypes to tease apart the structure.