Add native NiFi Record support

[stevedlawrence]

The current NiFi processor creates XML or JSON flowfiles. Ideally, we woudl have a more tightly integrated processor which takes a DFDL schema for data and hands off a NiFi data Record, described by the DFDL schema, so that its native access and manipulation of data can be applied.

There is an initial implemented in the dev branch that is mostly complete. We need to take another look at this branch and determine if any additional changes are needed and see if it can be merged into master. As well as document any limitations that Records have.

Looking through the pull request should give some insight on additional changes that may be neeed before merging: #1