Add log Av grid spacing option

[karllark]

Log A(V) spacing enabled. This is picked by add adding a "log" as the 4th element of the vector defining the A(V) grid. E.g., [0.01, 10.0, 0.1, "log"] to get a logAv spacing of 0.1 between 0.01 and 10.0.

This required adding in grid weights for all the dust parameters. Previously, these were always the same given a linear grid was used implicitly.

[codecov]

Codecov Report

Attention: Patch coverage is 87.87879% with 8 lines in your changes missing coverage. Please review.

Project coverage is 41.98%. Comparing base (487f14a) to head (8754af2).

Files with missing lines	Patch %	Lines
beast/physicsmodel/model_grid.py	37.50%	5 Missing ⚠️
beast/tools/verify_beast_settings.py	75.00%	2 Missing ⚠️
beast/physicsmodel/grid_and_prior_weights.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #823      +/-   ##
==========================================
+ Coverage   41.97%   41.98%   +0.01%     
==========================================
  Files         108      108              
  Lines       10288    10302      +14     
==========================================
+ Hits         4318     4325       +7     
- Misses       5970     5977       +7     

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