[#52] use interfaces to define DAModel
[#52] add Preconditions.checkArgumen(boolean, String)

add method checkArgument which supports specifying the IllegalArgumentException message
add javadoc on methods
[#51] fix DAImport, import type in unamed package is actually illegal

remove method DAImport#isDefaultPackage() which actually does not make sense as it is a compile error to try and import a type from the unamed package
make DAImportImpl static factory method raise a IllegalArgumentException when specified DAName to create the import from has no package
[#51] enforce DAName can not be empty
[#52] add javadoc to interfaces + move Impl class to impl subpackage
[#52] separate interface from class for DASourceClass