Move "Unit Test(s)" from optional to compulsory

[mikel-egana-aranguren]

In section 5, "Service Metadata", unit tests are optional items. I think that they should be compulsory metadata items instead (altogether with inpout OWL class, output OWL class, parameter OWL class), since unit tests are vital for building proper inputs to SADI services. Thi is so because building an input instance following the axioms of the input class is not always a simple process, and it is much easier to simply "copy" the predicates of the input on the unit test.