IFML — module defs

The spec misses the concept of ModuleDefinition, so one can use a module but not define it.
Module definitions should come with a packaging mechanism too.

Add ModuleDefinition and Module Package. Module renamed as ModuleDefinition (as a NamedElement); added Element, PortDefinition and ModulePackage in metamodel, with composite pattern. Added respective descriptions. Modules refer to ModuleDefinitions. Flows in PortDefinition can only be connected to elements within the ModuleDefinition. Flows in PortDefinition can only be connected to elements outside the Module. The set of Parameters of the Port is derived from the set of Parameters of the corresponding PortDefinition.
Created new general clause for describing the new concepts and showing the metamodel fragment. Created all the specific clauses needed for describing the new concepts.