Metasconstraints Part 1 - XML and Annotation Models (#254)

* Added a bunch of Javadoc comments.
Added support for flag and field instances to provide a different default value than the definition.
Implemented experimental support for let statements in constraints.
Refactored constraint parsing in the databind module.
Removed some unused code.

* Fixed maven warning caused by duplicate plugin declaration in pluginManagement.
Completed support for default value support on instances.

* Unified external constraint processing through use of an ExternalConstraintsModulePostProcessor.
Completed initial support for the MetaConstraintLoader. Still need to implement a true Metaschema-based Metaschema model to get constraint scope targeting working correctly.

* Added the start of a Metaschema Metaschema implementation by adding tests around parsing a Metaschema Metaschema. The full metaschema won't be parsed until a solution to usnistgov/metaschema#228 is implemented.
Added a new databind-metaschema/pom.xml Maven module as a placeholder for new parsing code based on generated Metaschema classes.

* Switched some tests using classpath resources to file resources to make testing more stable.
Refactored model interfaces to reduce boilerplate implementations.
Added support for choice groups in the XML parsed model and code generator.
Moved IAtomicValuedItem to a more sensible location.

* Refactored annotations to consolidate ValueConstraints, AssemblyConstraints, and GroupAs as child annotations of the core BoundAssembly, BoundField, BoundFlag, and BoundChoiceGroup annotations.
Cleaned up some compile and PMD warnings. Added some Javadocs.

* Silenced some compile warnings related to m2e in eclipse. Fixed some null reference and code style warnings. Also minor reformating.