Add support for nested fields and repeating groups

[bikeshedder]

I currently don't own any SunSpec compliant hardware that uses nested fields and/or repeating groups. I'd like to implement this feature to make this crate 100% SunSpec compliant.

If anyone could send me a dump of such data I'd be happy to implement this.

[bikeshedder]

My device does support Model 712. I've started working on a PR adding groups with the data my device reported:

• Add support for nested and repeating groups #5