Allow design-grid resolution to accept a tuple

[mawc2019]

This PR allows the resolution argument in filters.py to accept a tuple, so that different resolutions along different directions can be supported. This PR also refactors the code in filters.py.

[codecov-commenter]

Codecov Report

Merging #2599 (9c48402) into master (cd9a7eb) will decrease coverage by 0.09%.
Report is 1 commits behind head on master.
The diff coverage is 86.44%.

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@@            Coverage Diff             @@
##           master    #2599      +/-   ##
==========================================
- Coverage   73.90%   73.81%   -0.09%     
==========================================
  Files          18       18              
  Lines        5293     5294       +1     
==========================================
- Hits         3912     3908       -4     
- Misses       1381     1386       +5     

Files Changed	Coverage Δ
python/adjoint/filters.py	77.20% <86.44%> (-2.49%)	⬇️

[oskooi]

Would be good to include a unit test for this feature. One suggestion would be to simply verify that defining a filter with the resolution tuple (res, res) produces the same result as passing in just res.

[mawc2019]

One suggestion would be to simply verify that defining a filter with the resolution tuple (res, res) produces the same result as passing in just res.

I have added a different but more stringent test, in which one of the elements of the resolution tuple is zero, which defines a 1d design pattern. Suppose a 1d design pattern is defined in the x direction. Whether this 1d design pattern is periodic in the y direction or not should make no difference in the filtered 1d patterns. Likewise, suppose a 1d design pattern is defined in the y direction. There should be no difference whether this 1d design pattern is periodic in the x direction. The new test function checks this identity based on lengthscale constraint functions.

[oskooi]

LGTM.