Increase speed of get_unique_bias for Characteristic objects

[LeoMurphyWM24]

Description

I streamlined the get_unique_bias function on Characteristic objects in plasmapy.diagnostics.langmuir.py. According to a (very basic) TimeIt, creating 200 Characteristic objects now takes 0.59 seconds instead of 12.02 seconds, a 20-fold improvement in speed. The primary change is restricting get_unique_bias to averaging only duplicated bias values, instead of taking averages for all data points, even redundant single-point averages on already-unique bias values.

Motivation and context

The get_unique_bias function in plasmapy.diagnostics.langmuir.py runs every time a new Characteristic object is created, which appears to include heavily-used operations like subtraction between existing Characteristics. Streamlining get_unique_bias saves a lot of time when calculating plasma diagnostics on thousands of shots (according to my Python profiler, get_unique_bias was responsible for 88% of my program's twelve-minute runtime, and it now completes in two minutes!)

Note: my very basic TimeIt is found on a Google Colaboratory document here. If anyone catches an error in my code, I will fix it and be eternally grateful. I have not yet created a changelog entry in case any further edits are suggested.

[github-actions]

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