Merge branch 'fixes2' into 'NIFTy_5'

fixing metric of sum operator

See merge request !308
11 jobs for NIFTy_5 in 18 minutes and 24 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build Docker
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build_docker_from_scratch

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  Test
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test_mpi

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passed #773354
test_serial

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80.0%
 
  Release
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pages

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  Demo Runs
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run_bernoulli

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run_curve_fitting

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run_getting_started_1

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run_getting_started_2

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run_getting_started_3

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passed #773357
run_ipynb

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  Deploy
passed pages:deploy

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