Improved CopyStructuredData() to handle more cases.

CopyStructuredData() only supported the use case of copying from a larger structured dataset to a smaller one. This was added to handle output initialization of imaging algorithms. Extended the method to support cases where the two structured data are of arbitrary extents as long as they overlap. This is useful in various situations. For example, when writing ghost cell data exchange code, one has to receive a small structured data containing the ghost data and then copy those values to the larger structured data. For this use case, I also added SetupForCopy() which sets up the mapping between input and output arrays without having to call CopyAllocate() which wipes out any existing data.