Support Composite and Repeating Fields As First Class Models

[Dixon Whitmire]

Support composite fields such as the 271's EB13 as "first class models". This in turn will support granular validations. This is a notable improvement over existing multi-value support which just treats a composite field as a single string.

I have "decorated" each component field in the code base so that we can find these easily. To find each component field search for "is_component = True" in the "segments" module in either x12/v4010/segments.py or x12/v5010/segments.py.

These changes will require revisions to this conditional block.