Limit FFTW compat to v1.6

[jishnub]

FFTW v1.7 introduces certain changes to the type of r2rFFTWPlan (these are improvements over those introduced in v1.6, my bad for not incorporating it all in that version). Unfortunately, this change has led to breakages here. This PR limits the upper bound of FFTW to v1.6, and I'll submit another PR with the changes for v1.7 compatibility.

It also removes the redundant test on v1.9, as v1 points to v1.9 now.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (8d92144) 81.76% compared to head (cbda4f0) 81.76%.

❗ Current head cbda4f0 differs from pull request most recent head 65524b9. Consider uploading reports for the commit 65524b9 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #220   +/-   ##
=======================================
  Coverage   81.76%   81.76%           
=======================================
  Files          15       15           
  Lines        2539     2539           
=======================================
  Hits         2076     2076           
  Misses        463      463           

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