[#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
[#31] report errors at method and/or annotation line

when an error is detected on the dedicated class, it is now reported at the line of the incriminated method or its annotation instead of being reported exclusively on the class line