added saref:FeatureOfInterest and saref:measurementMadeBy

Changes implemented according to ETSI_TS_103_410-6 (SAREF4AGRI): From the development of the SAREF4AGRI ontology it has been observed the need for including the saref:FeatureOfInterest concept. Due to the general nature of such concept and its potential need in other extensions, for example SAREF4CITY, such concept is assumed to be included in SAREF 3.0 ontology. In the same line, the property saref:measurementMadeBy is proposed to be included into SAREF as inverse of saref:makesMeasurement to complete such model. Closes #40 (closed).