Merge branch 'integration_operator' into 'NIFTy_6'

Integration operator

See merge request !464
10 jobs for NIFTy_6 in 8 minutes and 22 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Build Docker
passed build_docker_from_cache #1081006

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  Test
passed test_mpi #1081008

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passed test_serial #1081007

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85.0%
 
  Demo Runs
passed run_bernoulli #1081014

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passed run_curve_fitting #1081015

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passed run_getting_started_1 #1081010

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passed run_getting_started_2 #1081011

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passed run_getting_started_3 #1081012

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passed run_getting_started_mf #1081013

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passed run_ipynb #1081009

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