upgrade the sos model to support several sensors per layer

[assefa]

In the current Mapserver SOS model, we have to specify 1 procedure (station) per
layer (using the sos_procedure metadata at the layer level). When the user wants
to use several stations, we eneded up duplication the layers with the procedure
being the only thing changing between layers 

The proposed change is to allow a use to specify a attribute from it's data that
can be ised as the procedure. Thus he could group into the same layer several
stations and the procedure could be extrcated from the data. 
To implement this the intention is to use an sos_procedure_item metadata.

[Assefa]

duplicate of ticket 2050