Permit to edit iDynTree::Model by removing Link or Joints

[traversaro]

For users that create iDynTree::Model instances by loading them from a file stored on a disk, it may be convenient to delete parts of the links or joints of the model to perform some processing or analysis. However, this will probably invalidate the mapping between link and joint and the relative indeces, so it should be clarified that such operations invalidate any Index or pointer previously obtained via getJoint,getJointIndex,getLink,getLinkIndex methods.