Improve performance in frequently called weighting functions

[jcurtis2]

• Added fast_pow to handle common exponents in aero_weight_num_conc_at_radius
• Removed array in aero_weight_array_num_conc_at_radius to avoid allocation costs

Urban plume scenario on Keeling saw a CPU reduction from 53.4s to 37.4s (with MOSAIC). Removing chemistry for a coagulation only case, this translates to approximately a 2x speedup.

[codecov]

Codecov Report

❌ Patch coverage is 70.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.40%. Comparing base (15b0ed4) to head (0580872).

Files with missing lines	Patch %	Lines
src/util.F90	62.50%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #221      +/-   ##
==========================================
- Coverage   77.45%   77.40%   -0.05%     
==========================================
  Files          55       55              
  Lines        9420     9435      +15     
==========================================
+ Hits         7296     7303       +7     
- Misses       2124     2132       +8     

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