Returns the number of elements in a block.
is - the index set
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.