Issue with system.get_net_impact() for LCA

Hi @yoelcortes

I hope you're doing well! I'm working on performing LCA with BLocS for one of my dissertation projects, but I've run into a slight problem with the built in `system.get_net_impact()` function.

When trying to run [`corn_LCFP_eval.py`](https://github.com/BioSTEAMDevelopmentGroup/BLocS/blob/main/blocs/corn_LCFP_eval.py) I get this error message (there is more to it but these are the last and most relevant lines):

File "/Users/daltonwstewart/Documents/Code/BLocS/blocs/corn_LCFP_eval.py", line 172, in get_total_CI_SD
LCA_3(system, NG_SD, cons_SD) +

File "/Users/daltonwstewart/Documents/Code/BLocS/blocs/corn_LCFP_eval.py", line 119, in LCA_3
total_emissions = system.get_net_impact(key=key) / system.get_mass_flow(cn.ethanol) # [kg CO2e/kg ethanol]

File "/opt/anaconda3/lib/python3.9/site-packages/biosteam/_system.py", line 2983, in get_net_impact
self.get_total_feeds_impact(key)

File "/opt/anaconda3/lib/python3.9/site-packages/biosteam/_system.py", line 2931, in get_total_feeds_impact
return sum([s.F_mass * s.characterization_factors[key] for s in self.feeds

File "/opt/anaconda3/lib/python3.9/site-packages/biosteam/_system.py", line 2932, in
if key in s.characterization_factors]) * self.operating_hours

TypeError: argument of type 'numpy.float64' is not iterable

It seems that the characterization factors for the feeds don't format in a way that the function can use. Can you take a look and see if it's the function or the way I've defined things in my file?

Thank you!
Dalton