Add a class that given a block sparse matrix m will compute
the product m'*m efficiently.
This code is refactoring and cleanup of the code in
CompressedRowSparseMatrix devoted to computing the inner product.
In that class, the code is mistakenly said to be computing
the outer product. It is also devoted to computing the inner
product of a CompressedRowSparseMatrix with itself.
This code works with BlockSparseMatrix objects instead, which
are simpler to deal with as they are better structured to handle
block sparse matrices.
Change-Id: I920fee1a396bb0fcae9e6f7e46a308c7391d21aa
Keir Mierle