Improve numerical robustness of JacoviSVD:  - avoid noise amplification in complex to real conversion  - compare off-diagonal entries to the current biggest diagonal entry: no need to bother about a 2x2 block containing ridiculously small entries compared to the rest of the matrix.
Improve numerical robustness of JacoviSVD:
 - avoid noise amplification in complex to real conversion
 - compare off-diagonal entries to the current biggest diagonal entry: no need to bother about a 2x2 block containing ridiculously small entries compared to the rest of the matrix.

pthulhu/eigen,pthulhu/eigen,pthulhu/eigen,pthulhu/eigen,pthulhu/eigen