Added GridProperty::multiplyWith( )
Added containsNaN() method to GridProperty
Added keyword MULTPV
Added post processor to GridProperty
Added postprocessor properties to scatter from top.

For the petrophysical properties PERM? and PORO only the top layer must
be specified, cells further down can be copied from the layer
above. This functionality is implemented with a GridProperty
postprocessor.
Added functionality to calculate PORV property.