Implement runtime warnings for functional property setters

[iosonopersia]

Setter methods that correspond to a functional property (e.g. hasTitle for a BibliographicResource) should either

• generate a runtime warning when an already existing value gets overwritten
• or allow the user to set a parameter flag to tell them not to update the value if another one is already set.

A similar functionality should be discussed and implemented also for "remover" methods that correspond to non-functional properties (e.g. remove_format for BibliographicResource): when the user doesn't specify a specific value to remove, the current implementation deletes all existing values. Instead, it should inform at runtime the user that he/she may have inadvertently forgot to specify the value to be deleted.