correct reordering of connection in sorted_space

- correction of 0796a6a3436bf5e5169efbcbd3188c13dbb7557c
- the remapping was not correct : -1 case not handled properly.
- strangely, the code passed tests (!), but was quite slow ...
- speed as before in simple tests.
correction : Yee trick & estimator

- quick exit is valid only for non estimator mode
- in estimator mode, however, one can have a similar trick, for pmin
  (commented out here).
- Check matrix < bound only in debug mode : in some rare cases, not
  satisfied due to rounding error for very small numbers.
Replace induced norm by Frobenius norm

Frobenius norm is not as tight, but a lot faster to compute.
This norm is used to replace the estimate of a subtree,
but only in the FullTrace mode, not in the Estimator mode,
in which this would raise detailed balance issues.

Performance gain seems rather small in simple, short tests.
To be looked at in longer runs.