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[bot]]

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 :warning:
beast/tools/verify_beast_settings.py	75.00%	2 Missing :warning:
beast/physicsmodel/grid_and_prior_weights.py	83.33%	1 Missing :warning:

Additional details and impacted files

```diff @@ 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 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.