Support for bulk addition of supplemental attributes

[daniel-thom]

We currently only support adding supplemental attributes one at a time. This is likely to be slow if there are thousands of attributes because each addition causes one row to be added to an SQLite table. We support bulk addition of time series for the same reason and could do the same for supplemental attributes.

[daniel-thom]

I made a prototype. Performance is ~3x faster with bulk additions. My benchmark adds 500k supplemental attributes to components.

• add 1 at a time: 24.378 seconds
• add 1000 at a time: 7.7 seconds

The code is straightforward and I'm not worried about that. There is a cost in user complexity for the bulk additions. The overall time involved here is not huge. Is it sufficient to justify the complexity (if users notice the docstrings)?