fix Rv_A calculation to proper Rv_B for user-input SMC-like dust model

[benjamin-gibson]

In beast/beast/physicsmodel/creategrid.py the function _make_dust_fA_valid_points_generator() had hardcoded the value of Rv_B to 2.74. This value is correct for the G03_SMCBar extinction curve but not for any other. This function now accepts the variable Rv_B from the user-selected extinction law.

[karllark]

Please fix the codestyle errors. I often just run the "black" auto-formatter to avoid any codestyle issues like this.

[codecov[bot]]

Codecov Report

:x: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review. :white_check_mark: Project coverage is 40.55%. Comparing base (b799489) to head (d819cce).

Files with missing lines	Patch %	Lines
beast/physicsmodel/creategrid.py	75.00%	1 Missing :warning:

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #846   +/-   ##
=======================================
  Coverage   40.55%   40.55%           
=======================================
  Files         107      107           
  Lines       10320    10321    +1     
=======================================
+ Hits         4185     4186    +1     
  Misses       6135     6135           

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