Refactoring CMDIDataImpl

[wowasa]

Reducing the information on the necessary information:

• we're splitting by type while extending CMDIDataBaseImpl to unite the in curation again, since we don't need to know the resource type there but only mime-type and URL. Therefore we should only implement CMDIData but not extend CMDIDataBaseImpl
• we're storing instances of ValueSet in CMDIDataImpl but we only need a view information of the ValueSet. Hence the add methods should already select the information we need for storage