Add api methods to fetch drugs by reference mappings - TRUNK-4144
Author:    Nicholas Ingosi Magaja <nicotwendelee@gmail.com>

post initial commit changes

looping through the collection

looping through the collection

improved javadoc

post code review
*Include only one method in the dao
*Filter the other method within the serviceimpl

making the drug the root and joining to the drugreferencemap

making the drug the root and joining to the drugreferencemap

linking to drugReferenceMap property and removal of alias to the drug property

checking for null concept source

Enhanced methods and unit tests

Added dataset for unit test

Added unit test dataset and the test methods
Follow up to add api methods to fetch drugs by reference mappings - TRUNK-4144