Fix for infinite loop in Brent's method

Fixes #861 (closed).

@mivkov Now that there are none of the debug messages from the reproduce_861 branch, I did notice quite a bit of rt_check_unphysical_state: Fixing unphysical energy case 4 | -2.368605e-09 | 7.137288e-04 2.072430e-03 1.103263e-03 | 7.929667e-02 messages. Do you think this a (related) issue?

Also should look into making the Newton-Rhapson iteration the default again as in @bvandenbroucke's shadowfax implementation?