Add data directory as variable in shablona namespace

[petebachant]

Instead of

import os.path as op
import shablona as sb
data_path = op.join(sb.__path__[0], 'data')

It would be nice to be able to use

import shablona as sb
data_dir = sb.data_dir

Some clever use of os.path and __file__ should make this easy enough, but I forget the exact syntax.

I think this will also require that users install in editable mode, unless data is copied to site-packages with -e omitted:

pip install -e .