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.
Merge pull request #645 from bska/fix-out-of-bounds-indexing

effectiveExtent: Fix out-of-bounds indexing