use banded matrix decomposition instead of dense LU for BBM-BBM (#170)

Author: ranocha

src/equations/bbm_bbm_1d.jl

@@ -371,7 +371,7 @@ function create_cache(mesh, equations::BBMBBMEquations1D,
     N = nnodes(mesh)
     M = mass_matrix(solver.D1)
     Pd = BandedMatrix((-1 => fill(one(real(mesh)), N - 2),), (N, N - 2))
-    D2d = (sparse(solver.D2) * Pd)[2:(end - 1), :]
+    D2d = (BandedMatrix(solver.D2) * Pd)[2:(end - 1), :]
     # homogeneous Dirichlet boundary conditions
     invImD2d = lu(I - 1 / 6 * D2d * K_i)
     m = diag(M)