This changes simplifies the specification of functionObjects in
controlDict and is consistent with the 'libs' option in controlDict to
load special solver libraries.

Support for the old 'functionObjectLibs' name is supported for backward compatibility.

functionObjects: Changed options 'outputControl' -> 'writeControl' and 'outputInterval' -> 'writeInterval'

for consistency with the time controls in controlDict and to avoid
unnecessary confusion.  All code and tutorials have been updated.

The old names 'outputControl' and 'outputInterval' are but supported for
backward compatibility but deprecated.

See $FOAM_ETC/caseDicts/postProcessing/README for details