feature values from RasterToVectorProcess

In the schema for vector features generated by RasterToVectorProcess the "code" property which records the value of the grid region bounded by a polygon feature is an integer. This is pretty useless, for instance, when vectorizing a grid coverage with floating point values between 0 and 1.

Change it to a double and change the name to "gridvalue" to make it more obvious where it's come from.