Add support for OPFData dataset format

[akuramshin]

Adding a parser for the OPFData dataset. This parser is largely based on the existing parser for matpower files.

[odow]

It is possible to use a smaller test example that isn't half a million lines?

[akuramshin]

It is possible to use a smaller test example that isn't half a million lines?

Sorry about that! Replaced with a smaller case file. Let me know if you would prefer an even smaller file