set_index for new coordinates

[mschrimpf]

When new coordinates are added to an assembly, they are not automatically added to the index.
For instance: assembly['region'] = 'neuroid', ['IT'] * len(assembly['neuroid']) will not allow assembly.sel(region='IT') but rather fail with a non-indexed error.
To fix (borrowing from @fksato): assembly.set_index(neuroid='region', append=True).