Patch: pack, unpack functions

[Stephen Smith]

<<10Sep.pack.patch>>
These changes add the pack/unpack functions requested by Blanca.
Tested on Linux with --messaging on 2 processors.
Reviewed by Scott Haney.

Log Message:
 * PackUnpack.h:
   - Added a pack/unpack function requested by Blanca.
   pack() returns a RefCountedBlockPtr to all the elements in a field
   (not including guard layers) that are local to a given processor.
   unpack() copies the data from the RefCountedBlockPtr back into the field.

These functions were tested in parallel which led to the following small
changes:
 * FieldEngine.ExrpEngine.h - added a non-const accessor for the engine
    (used by reductions of remote engines)
 * FieldOffset.cpp - added blockAndEvaluate before values are checked
 * LocalPatch.cpp - added a test of pack/unpack
 * Arrays.h - added include of Functions/PackUnpack.h

10Sep.pack.patch
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