Prepare SparseFEM benchmark to changes to find() on Julia master

find() will soon return cartesian indices, so this benchmark needs
to convert them to linear indices. Unfortunately, LinearIndices
currently does not allow indexing with an array, so we need to use map().

src/problem/SparseFEM.jl

@@ -20,7 +20,7 @@ end
 
 # timing of assembly, slice and solve
 function perf_sparse_fem(N)
-    Ifree = get_free(N)
+    Ifree = map(i -> LinearIndices((N, N))[i], get_free(N))
     # assembly
     A = fdlaplacian(N)
     # boundary condition