change default fftw plan
Description
The current default for the FFTW_PLAN (measure) introduces numerical fluctuations between runs, and therefore we change the default to estimate, which slightly increases the runtime for longer runs (but has shorter overhead). Users can still change the plan back to measure, if this is an issue. The numerical noise, on the other hand, is problematic for the testsuite.
News snippet
Change the default plan of the FFTW
Checklist
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I have checked that my code follows the Octopus coding standards -
I have added tests for all the new features added in this request.