Add performance comparisons to docs

[nickrobinson251]

Now we support v33 data, we can compare against other packages (which seem to only support v33 data), and we might want to add performance comparisons to the docs (see e.g. #47 (comment) for a rough indication).

We'd want to be a bit careful, as it's possible the other packages do other stuff i.e. they don't just parse the data as given, but may also do some "fixing" or "transforming" of the data... but we'd need to investigate to be sure.

Also performance of the parser will depend on what categories make up the data.

One option would be to have a grouped bar chart, comparing perforance on a range of different files (e.g. the files in https://github.com/NREL-SIIP/PowerSystemsTestData/)

(This issue is about performance compared to alternative packages. The issue about tracking the performance of this package itself over time is #54)