Returns the number of elements in a block.
PetscErrorCode ISGetBlockSize(IS is,PetscInt *size)
|size ||- the number of elements in a block
This is much like the block size for Vecs. It indicates that one can think of the indices as
being in a collection of equal size blocks. For ISBlock() these collections of blocks are all contiquous
within a block but this is not the case for other IS.
ISBlockGetIndices() only works for ISBlock IS, not others.
ISBlockGetSize(), ISGetSize(), ISCreateBlock(), ISSetBlockSize()