bugfix: avoid Eigen assertion in FindMembrane with cmake flag -DOPTIMIZE=0

Eigen does not like to compute "Thin" U and V matrices in JacobiSVD when
matrix size is fixed at compile time. This triggers an assertion (optimized
away with -DOPTIMIZE=1). Since we're talking of a quadratic matrix here,
"Thin" and "Full" make no difference.