Note: This seems to have introduced some bugs. ...to be corrected soon.

The continuous restartability still does not seem to work correctly.

This implies the reduction of required metric calls in the broyden code path
from 4 to 2. This is now equivalent to the number of metric calls in the
old broyden code path.

This is not yet fully functional and not yet activated in the code. To activate it
the broyden call in "mix" has to be replaced by a broyden2 call.