More effective linear solve.

Now that the mass matrix is symmetric positive definite, switch from Belos Block
GMRES to Block CG. Tried two preconditioners: "Diagonal" (Jacobi) and
RILU(0). Both are quite effective on the model problem: for a tolerance of
1e-12, RILU(0) yields 8 iterations and Diagonal ~40. A Matlab prototype suggests
RILU(0) may scale to many processors with overlap=0. But have to wait for bigger
problems to investigate further.

While implementing the linear solve code, also cleaned up the code in several
spots.