Add new IE result attribute/field to store the intersection Area

[eblondel]

The intersection Area should be persisted as new attribute in the target IE result. If not retrieved by the IntersectionFeatureCollection WPS process, this should be recomputed applying SRCOVLPCT * SRCAREA

Data type of this new attribute: Number

This new attribute should be added to the attributeMappings in ie2rtms flow, in order to store it in RTMS intersection table