Move to plurial some getters that returns a collection

It seems that each getter returning a collection is plurial. For example in Citation, the method getDescriptiveKeywords() or getResourceSpecificUsages(), in MetaData getContacts() ...

Here is a list of getters that I think could be in plurial:

• Identification
  + getResourceMaintenance()
  + getResourceFormat()
• DataIdentification
  + getLanguage()
  + getExtent()
• Constraints
  + getUseLimitation()
• Element
  + getNamesOfMeasure()
• Georeferenceable
  + getParameterCitation()
• Citation
  + getPresentationForm()

Here I join a patch where I depreciate the old methods for the new ones that have a name in plurial.