Tutorials: Added matrix-free use of iterative solvers.

Example works for all three backends.
Required a couple of internal reorganizations of prod(), handle(), and size().
Interface requirement for user-provided operators:
 - member function apply(x, y); to compute y = A * x;
 - member function size1(); to return the length of y.
The second interface requirement is technical, but will be needed
for matrix-free applications of rectangular matrices later.

Resolves #74.