effectiveExtent: Fix out-of-bounds indexing

Commit 96cf137 introduced support for Peaceman index calculation that honoured general completion directions (`X`,`Y`,`Z`).  This was accomplished through a permutation index that reordered the permeability and geometric extent components according to a local coordinate system along the completion.

In a complete breakdown of logic, however, the d-component extent vector was indexed as though it were a d-by-d matrix.  This commit restores sanity to the processing.

Pointy hat: @bska.

This resolves OPM/opm-autodiff#194.