Extend VectorToRasterProcess options

At the moment the VectorToRasterProcess class uses an attributeName parameter to get a numeric value from each feature that is used for the corresponding cell values in the output raster.

Add the ability to pass an org.opengis.filter.expression.Expression parameter that can be evaluated to a numeric value to allow for feature value recoding, interpolation etc. as well as generating unique integer ID values from featureIds so that the output raster can be linked to a table of other attributes.