Add matfaust.lazylinop.diag.

wrapper/matlab/+matfaust/+lazylinop/diag.m

+%=============================================================
+%> @brief Constructs a diagonal LazyLinearOp.
+%>
+%> @param v a vector for the diagonal.
+%> @param k (int, optional) diagonal to place the vector on. Default is 0 (main diagonal). Negative integer for a diagonal below the main diagonal, strictly positive integer for a diagonal above.
+%>
+%> @retval the diagonal LazyLinearOp.
+%>
+%=============================================================
+function DL = diag(v, varargin)
+
+	import matfaust.lazylinop.*
+    p = inputParser;
+	validK = @(k) isscalar(k) && 0 == k - floor(k);
+	addOptional(p, 'k', 0, validK)
+
+	parse(p, varargin{:})
+	k = p.Results.k;
+
+    m = numel(v) + abs(k);
+    v = reshape(v, numel(v), 1);
+
+    function y = matmat(x, v, k)
+        if isLazyLinearOp(x)
+            DL = diag(v, k) * x;
+        else
+            if k > 0
+                y = v .* x(k+1:k+numel(v), :);
+                y = [y ; zeros(k, size(x, 2))];
+            elseif k < 0
+                y = v .* x(1:numel(v), :);
+                y = [zeros(abs(k), size(x, 2)) ; y];
+            else % k == 0
+                y = v .* x(1:numel(v), :);
+            end
+        end
+    end
+	DL = LazyLinearOperator([m, m], 'matmat', @(x) matmat(x, v, k), 'rmatmat', @(x) matmat(x, v, -k));
+end