specify default no smoothing for MaterialGrid constructor

[oskooi]

#1780 added support for subpixel smoothing of the adjoint gradient and also set smoothing as the default for the MaterialGrid. (#1886 fixed a bug to permit no smoothing via do_averaging=False of the MaterialGrid.) Since subpixel smoothing of the adjoint gradients is currently an experimental feature which can, under certain circumstances, significantly slow down the runtime performance of the get_gradient function of the adjoint-solver module (compared to no smoothing), it is probably better to use a default of no smoothing for the MaterialGrid. The default setting can be changed once this feature is more mature and the performance is better tested (via e.g. #1855).

(Since forward simulations that do not involve computing the adjoint gradient do benefit from the improved accuracy of subpixel smoothing of the MaterialGrid without a noticeable degradation in runtime performance, it could be useful to have a separate flag for subpixel smoothing in the adjoint solver. This way, the do_averaging property of the MaterialGrid can still be True by default and overwritten to False by the adjoint solver.)

[codecov-commenter]

Codecov Report

Merging #1903 (623a2d4) into master (accaf4f) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1903      +/-   ##
==========================================
+ Coverage   73.13%   73.15%   +0.02%     
==========================================
  Files          17       17              
  Lines        4924     4924              
==========================================
+ Hits         3601     3602       +1     
+ Misses       1323     1322       -1     

Impacted Files	Coverage Δ
python/geom.py	93.87% <ø> (ø)
python/simulation.py	76.85% <0.00%> (+0.04%)	⬆️