[!230] [bugfix] Use submatrix index vector to implement O(1) look-up

Merge branch 'feature/fix-quadratic-umfpacksetup' into 'master'

ref:core/dune-istl Looking up if a column index is contained in submatrix could degenerate to accumulated O(n) complexity for each row in the old implementation. By setting up a look-up before, this can be improved to O(1). Fortunately such a vector is constructed anyway so we can simple use it after minor reordering of code.

This should fix #54.

See merge request core/dune-istl!230

Closes #54 (closed)

(cherry picked from commit 0f4b78a9)

22a5ae5f [bugfix] Use submatrix index vector to implement O(1) look-up