rabi flop fit failing

[hartytp]

This may be related to recent tweaks to the FFT heuristic.

Example dataset

x = array([6.20335041, 6.20667686, 6.21000332, 6.21332977, 6.21665623,
       6.21998268, 6.22330913, 6.22663559, 6.22996204, 6.2332885 ,
       6.23661495, 6.2399414 , 6.24326786, 6.24659431, 6.24992077,
       6.25324722, 6.25657368, 6.25990013, 6.26322658, 6.26655304,
       6.26987949, 6.27320595, 6.2765324 , 6.27985885, 6.28318531])
y = array([[0.14, 0.18, 0.1 , 0.04, 0.04, 0.06, 0.08, 0.2 , 0.18, 0.5 , 0.64,
        0.72, 0.9 , 0.92, 1.  , 0.92, 0.88, 0.74, 0.58, 0.42, 0.22, 0.14,
        0.1 , 0.06, 0.1 ]])