[Aurelius] Add Converters from Model Object to Complete PFA Doc

[StevenMMortimer]

Currently, models generated with Aurelius cannot be converted directly from their state as an object into a valid PFA document. Is it possible to add S3 wrapper methods for model object types (e.g. glm, lm, etc.) that convert these models directly to a completely valid PFA document? This would decrease time in having to construct these simple models by hand from their object components as done in the tutorial:

https://github.com/opendatagroup/hadrian/wiki/Aurelius-glm

[jpivarski]

If you know how to do it--- i.e. how to extract the relevant parameters from the S3 objects--- then you know how to expand this function. If you can do that, please make a pull request.

...

You're faster than I am. You just submitted a pull request. I think it should be accepted, but someone from ODG will have to do it.

[StevenMMortimer]

@jpivarski Thanks Jim! That sounds good. I started using PFA more from R and I think some wrapper functions plus documentation would make it easier for R users to adopt.