rename has_parameter to has_parameters

[tramyn]

A protocol_interface can have zero or more parameter(s). The current syntax to get parameters from a protocol interface implies that there is alway one parameter (ex: protocol_interface.has_parameter) tied to an instance of a protocol interface. Since this is not the case, I would like has_parameter rename to has_parameters. By doing so, this makes it clear that I can expect more than one parameters to be listed in a protocol interface.