Merge branch 'add_nfft' into 'NIFTy_5'

Add NFFTOperator

See merge request !304
9 jobs for v5.0.1 in 8 minutes and 42 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build Docker
passed #768568
build_docker_from_cache

00:00:16

 
  Test
passed #768570
test_mpi

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passed #768569
test_serial

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  Demo Runs
passed #768575
run_bernoulli

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passed #768576
run_curve_fitting

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passed #768572
run_getting_started_1

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passed #768573
run_getting_started_2

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passed #768574
run_getting_started_3

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passed #768571
run_ipynb

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