incorrect incoherent scattering length from periodictable

We appear to be using the wrong property when looking up incoherent scattering lengths from the `periodictable` module, such as when we calculate the incoherent scattering length:
https://github.com/MDMCproject/MDMCv0.2_pilot/blob/f9d48cefbf27d4a7b2492a76f5c3b30dbc80dcdd/MDMC/trajectory_analysis/observables/fqt.py#L939
The property `b_c_i` refers to the "imaginary bound coherent scattering length", see https://github.com/python-periodictable/periodictable/blob/0b8ba54b6fe9ff5ace0dba0eef23cf485962f5a8/periodictable/nsf.py#L363 for reference.

Start at MDMC/trajectory_analysis/observables/fqt.py line 939 and compare the scattering-length property with the definition in periodictable/nsf.py around line 363. Confirm which property represents incoherent scattering length and update the lookup; done means the calculation no longer uses b_c_i for this quantity.

Written by the indexing model from the issue text.