Fitting for scattering parameters is broken

[kmjc]

I ran into a couple of errors in derivative.py trying to fit for scattering

a) deriv_model_wrt_scattering_index
I'm not sure these two lines https://github.com/CHIMEFRB/fitburst/blame/57a86a8ae747b08071be4e4ab3f76f325294a293/fitburst/routines/derivative.py#L747-L748 need to exist since spectrum is overwritten immediately below? If they are needed then line 747 throws a couple of NameErrors

I think you also need to add

burst_width = parameters["burst_width"][component]

or something similar since burst_width throws a NameError too.

b) deriv2_model_wrt_burst_width_scattering_index
log_freq = np.log_freq_ratio -> log_freq = np.log(freq_ratio)

[emmanuelfonseca]

this has been fixed in #84.