Gideon :Added the configuration parameter nBestFormat to the JoshuaConfiguration, in preparation of
adapting the JoshuaDecoder to enable outputing a sparse features representation nBestList.
This is required in order to be compatible with the more complex tuning schemes (2,3,5) of PRO.
Also moved the correctness check for the value of nBestFormat form PROCore to a static method in JoshuaConfiguration
in order to avoid code duplication (i.e. The DRY : Don't repeat yourself principle).