With this change each functionObject provides the list of fields required so
that the postProcess utility can pre-load them before executing the list of
functionObjects. This provides a more convenient interface than using the
-field or -fields command-line options to postProcess which are now redundant.
Currently these deleted function declarations are still in the private section
of the class declarations but will be moved by hand to the public section over
time as this is too complex to automate reliably.
