Provide foldl+r( [in,out] Matrix<D>, [in] Matrix<T>, Monoid )

[byjtew]

This issue concerns the implementation for the reference+omp, hyperdags and distributed backends.

The foldl and foldr primitives, which are currently not implemented in any backend appear to be crucial methods that need to be implemented.

Documentation draft for masked foldl (foldr is implicit):

/**
 * Scales, or \em folds, a matrix into a matrix, using a constant 
 * matrix to perform element-wise operations against.
 *
 * @tparam descr     The descriptor to be used (descriptors::no_operation if
 *                   left unspecified).
 * @tparam Monoid    The monoid to use.
 * @tparam IOType    The type of the elements in the input and output
 * 					 ALP/GraphBLAS matrix \a A.
 * @tparam MaskType  The type of the elements in the supplied ALP/GraphBLAS
 *                   matrix \a mask.
 * @tparam InputType The type of the input matrix \a B.
 *
 * @param[in,out] A     Any ALP/GraphBLAS matrix, which will be scaled.
 * 					    Prior value will be considered.
 * @param[in]     mask  Any ALP/GraphBLAS matrix.
 * @param[in]     B	    Any ALP/GraphBLAS matrix, which will be used to
 * 						scale \a A.    
 * @param[in]   monoid  The operator under which to perform the element-wise
 * 						scaling.
 *
 * @return grb::SUCCESS  When the call completed successfully.
 * @return grb::MISMATCH If A, mask and B don't have the same dimensions.
 *
 * @see grb::foldr provides similar in-place functionality.
 * @see grb::eWiseApply provides out-of-place semantics.
 *
 * \parblock
 * \par Valid descriptors
 * - descriptors::no_operation: the default descriptor.
 * - descriptors::no_casting: the first domain of
 * 	 	\a op must match \a IOType, the second domain of \a op
 * 		match \a InputType, the third domain must match \a IOType.
 * - descriptors::transpose_left: mask^T will be considered 
 * 	 	instead of \a mask.
 * - descriptors::transpose_right: B^T will be considered 
 * 	 	instead of \a B.
 * - descriptors::invert_mask: Not supported yet.
 *
 * \note Invalid descriptors will be ignored.
 *
 * \endparblock
 *
 * \par Performance semantics
 * Each backend must define performance semantics for this primitive.
 *
 * @see perfSemantics
 */
template<
	Descriptor descr = descriptors::no_operation,
	class Monoid,
	typename IOType, typename MaskType, typename InputType, 
	typename RIT_A, typename CIT_A, typename NIT_A,
	typename RIT_M, typename CIT_M, typename NIT_M,
	typename RIT_B, typename CIT_B, typename NIT_B,
	Backend backend
>
RC foldl(
	Matrix< IOType, backend, RIT_A, CIT_A, NIT_A > &A,
	const Matrix< MaskType, backend, RIT_M, CIT_M, NIT_M > &mask,
	const Matrix< InputType, backend, RIT_B, CIT_B, NIT_B > &B,
	const Monoid &monoid = Monoid(),
	const typename std::enable_if< 
		!grb::is_object< IOType >::value &&
		!grb::is_object< InputType >::value &&
		!grb::is_object< MaskType >::value &&
		grb::is_monoid< Monoid >::value, void
	>::type * const = nullptr
) {}